Per 
SCP-5402
Special Containment Procedures: SCP-5402 is to be kept in a small, opaque box in Site 12's compact anomaly storage unit when not in use. When in use, at least three staff without firsthand knowledge of SCP-5402's existence must be available on call. Staff are advised against blinking or otherwise breaking eye contact with SCP-5402 whilst handling it. SCP-5402 is to be handled only with the permission of staff with Level 2 security clearance or higher.
Description: SCP-5402 is a standard iron shot put ball, roughly 13 cm in diameter, which is only perceptible to those who are unaware of its existence. Any individual who can recall SCP-5402 is incapable of perceiving or interacting with it in any way. Upon initial exposure to SCP-5402, subject may see, hear, feel, or directly manipulate it until direct contact is lost. Further interaction with SCP-5402 is possible only if direct contact is sustained from first exposure (e.g., if subject can feel SCP-5402 in their hands, they will still be able to see it even after breaking eye contact prior).
Images, video recordings, and audio recordings of SCP-5402 will all produce the same anomalous effect as encountering it directly. Secondhand knowledge of SCP-5402's existence (for instance, hearing a recollection of SCP-5402 from another person) produces no such effect.
SCP-5402 does not interfere with one's memory. Subjects are perfectly capable of remembering SCP-5402 after contact, even while still incapable of perceiving it. There are no discernible effects toward short or long term memory of exposed persons, regarding SCP-5402 or otherwise. Physically altering SCP-5402 (e.g., moving it, painting it, etc.) without subject's awareness does not change one's ability to perceive it.
When tested against subjects with short term memory loss, they were often able to perceive SCP-5402 after contact had been broken and reestablished— presumably because they had forgotten SCP-5402 existed. All requests to test SCP-5402 against infants to study the development of object permanence have been denied.
Any stress or confusion regarding the supposed disappearance of SCP-5402 is considered to be a rational emotional response and is not anomalous.
Document 5402-1: Test Log 5402-D
Test 5402-D - [02/04/2019]
Overseeing Researchers:
Dr. Otho
Subjects:
Subjects D-2020, D-8619, D-4095, D-3323
Procedure:
1. Subject is blindfolded and SCP-5402 is placed into their hands. Blindfold is then removed, and subject is instructed to memorize the object in their hands. Subject is then blindfolded again, and SCP-5402 is taken away. This process is repeated with five more mundane objects.
2. Subject is distracted for five minutes with a menial task, then asked to recount the five objects they observed.
3. Subject is presented with eight opaque boxes, each with a hole large enough to fit their hand through. Three of these boxes contain one of the mundane objects, one contains SCP-5402, and the other four are empty. The box containing SCP-5402 also contains a small camera being monitored through an outside video feed.
4. Subject places their hand in each box and is instructed to describe which object they feel in the box, if any. Subject is then asked to report which objects were missing from the boxes.
Results:
Subjects reported SCP-5402 as missing every time, despite SCP-5402 always being present in one of the boxes. Footage from the camera feed shows subjects' hands moving directly through SCP-5402.
Notes:
Clearly, once someone is made aware of SCP-5402, it becomes imperceptible to them. Still not sure what the implications of this are. My first thought was that this thing had an antimemetic property about it, but I am no longer convinced by that. It seems more to me like an object that can only be discovered once before it vanishes: something you only get to see one time.
What still puzzles me is the fact that it's not the mere perception of SCP-5402 that is hindered, but the ability to directly interact with it at all. How could their hands occupy the same space as SCP-5402 without ever moving it?
Document 5402-2: Test Log 5402-E
Test 5402-E - [03/07/2019]
Overseeing Researchers:
Dr. Otho
Subjects:
Subjects D-2020, D-8619, D-4095, D-3323
Procedure:
1. The same subjects as tested in 5402-D are gathered for an unrelated experiment to ensure they are not primed to recall the events of the previous test.
2. Partway through this test, subject is instructed to reach inside of opaque bag containing SCP-5402 and describe what they feel.
3. Original experiment resumes as planned.
Results:
All subjects report feeling a hard, spherical object in the bag.
Notes:
All of them being able to feel it leads me to believe that whether an individual has encountered SCP-5402 before has nothing to do with its perceptibility. Rather, since none of them had any reason to think SCP-5402 was special in that test from a month ago, they all forgot about it. It is safe to conclude that it is the memory of SCP-5402 that prevents it from being observed.
Also of note: the people who carried the bags were the ones to put SCP-5402 in them in the first place. This means that despite being aware of SCP-5402, they were still able to move it indirectly.
Document 5402-3: Notes from Dr. Otho, 03/08/2019
[03/08/2019]: After the last test, I've taken some strange comfort in going and picking up the box 5402 is being contained in. I can't feel its weight, but it's nice to know that I'm still able to interact with the thing I'm supposed to be researching, even indirectly. Watching new research assistants see it for the first time feels like a bad practical joke. It's awful, knowing I can never see the thing I'm devoting so much thought to because I'm thinking about it. It's starting to get to me.
Document 5402-4: Test Log 5402-F
Test 5402-F - [03/12/2019]
Overseeing Researchers:
Dr. Otho
Subjects:
Subjects D-8111, D-6902
Procedure:
1. Subject stands in the center of a room with ballistic force-resistant walls, monitored by a camera and microphone. A small hydraulic cannon is mounted in the wall directly behind the subject.
2. Subject retrieves SCP-5402 from opaque bag and is instructed to familiarize themselves with it for 10 seconds before returning it to the bag.
3. SCP-5402 is loaded into the hydraulic cannon and shot at subject at 500 m/s.
Results:
Subjects report hearing the noise of the cannon, but no sensation of being hit by SCP-5402. Subjects sustain no bruising or injury despite the projectile having more than enough force to break bones. Footage from the camera shows SCP-5402 passing right through subjects and hitting the floor on the other side of them. The audio recordings pick up the noise of SCP-5402 hitting the ground, but subjects do not report hearing any noise other than the cannon.
Notes:
It has been established that interaction with SCP-5402 is impossible for an individual who is conscious of its existence, but this confirms that the reverse is true as well: SCP-5402 cannot interact with any individual who remembers it. They can occupy the same space, but they cannot affect each other. The only conclusion I can draw from this is that after you are aware of SCP-5402, it completely ceases to exist in your physical reality.
Document 5402-5: Journal Entries from Dr. Otho, 03/13/2019 - 03/18/2019
Excerpt 5402-5A:
[03/13/2019]: 5402 might have some revolutionary implications for the study of parallel planes of reality. What if our notion that this thing doesn't follow the laws of our universe is wrong? If 5402 was publicized, it could turn the whole field of cosmology on its head. It could open the door to a whole new world of scientific theory. The discovery that two disparate things can occupy the same quantum space as one another, anomalous or not, flies in the face of our modern understanding of quantum mechanics. It should be impossible, but I saw it with my own…
I mean, I was at least told that it happened. Other people saw it, multiple people with the exact same accounts. It was caught on camera. The fact that I didn't get to see it myself is irrelevant.
Excerpt 5402-5B:
[03/15/2019]: The heart of research is supposed to be observation. How can I observe something which no longer exists to me? An object that can live in your memory or your reality, but never in both… That's just a fantasy, isn't it? Does SCP-5402 truly reside in a plane parallel to my own, or does it only exist in my head? Isn't the most plausible explanation that I'm just deluded by false memories?
If that's the case, then why can other people see it? Why does the evidence corroborate my impossible conclusions?
Excerpt 5402-5C:
[03/15/2019]: I just want to see it again. Just once, so I know it's real.
Excerpt 5402-5D:
[03/18/2019]: Every day feels more and more like this whole facility is just entertaining my delusion. I know what the evidence is telling me. I'm researching something that doesn't exist.
Addendum [03/22/19]: Following the psychological evaluation of former overseeing researcher Dr. Matt Otho, Site Director █████ has prohibited further testing of SCP-5402 indefinitely.
