Coworker is intimidating me by taking items from teammates’ desks and putting them on mine

tldr

Unless you're extraordinarily meek, then usually the first response expected is self-help, such as telling him to stop his pranks and leave you alone, repeat this warning on a few further occasions, and then escalate to your immediate manager.

Longer read

So he's a mild prankster, perhaps (and this is unclear...) a slightly malevolent one.

Moving two minor items between desks (especially things that are clearly noticed missing at the source, and clearly out of place at the destination), and making a suggestion for an April Fool's prank that you didn't take up, isn't particularly concerning behaviour.

His suggestion that you "ruin all the measurement results" of your colleague, obviously it may just have been a bad idea that didn't withstand scrutiny. I assume there was no serious pressure applied, just an ordinary level of verbal urging.

The usual trick with a prank is to either cause surprise or very mild annoyance, or waste a very small amount of effort (a few minutes, certainly not hours or more). Sometimes the prankster must participate, for example by "discovering" a "stolen" item, or "noticing" things that have gone wrong, but doing so in a certain way that shortly reveals that they are the real perpetrator. Obviously there's a skilfulness to it all, and done correctly it's supposed to be fun and promote social cohesion, not destroy trust and sow paranoia.

Up until then, he had been making little suggestions to try to get me to isolate myself from my team.

Such as? It must be more than suggestions for minor pranks.

He has also been egging on some of my more “wacky behavior” (I’m autistic/adhd and can sometimes be a bit much), likely to tarnish my reputation.

Which is again consistent with a prankster, who may needle people into eruptions to which they are prone, or promote silliness.

(I’m afraid he’ll isolate me from my friends, so I haven’t told them or left the chat)

Then you've already self-isolated from your friends, if indeed they are your friends.

You also say this guy was your friend, but you are now scathing about him, without being specific about what makes him "extremely manipulative" or "creepy", especially in regards his workplace conduct.

There is a security camera that may have captured at least one instance of him tampering with my desk

Is there actually a serious question from anybody about whether he has tampered, or serious suspicion on you? Usually pranksters quickly acquire a reputation.

If I was HR, I'd be liable to interpret this whole situation as two social friends-turned-foes bringing their personal disputes or dislike into the workplace.

You really do have to substantiate it with more than "he pranked me twice by moving an item from elsewhere to my desk, and twice suggested I also do silly things, some/all of which occurred when we were friends".

To say there has been "intimidation" sounds completely preposterous based on what you have articulated - at best there has been "annoyance".

I suspect this guy likes you romantically and has no idea how to express his feelings. So he might be on the spectrum as well, or at least immature.

There is nothing wrong with you mentioning to your team that he desired to pull that prank about the ruined measurements. Tell them you would never do such a thing. This will bring you closer to your team and make sure you make it a bit casual. Do not mention who unless they really pressure you.

If you go to security, HR, or just talk to your team talk about the stuff moving but not who you suspect is doing it. You are probably right, but you do not know for sure. But you do know what is happening.

So when it happens return the items and say something like "Well the prankster is at it again, I ended up with a photo of your wife on my desk. Sorry about that, but I do not know what to do." You might even offer to clean the item.

If its your team or security/HR that figures out who is doing this you will be seen as blameless. If you tell them, it could backfire on you.

As far as your friends group, same kind of thing. When he does something creepy. Just say, wow that is kind of creepy or casually call out the behavior. Let them all arrive to the conclusion that he is who he is. There are probably others that feel the same way.

And clearly, he is not your actual friend.

And own your "whacky behavior", it makes you unique and are who you are.

I'll play devils advocate here:

There is nothing you have told us that indicates he is at fault. Everything you wrote is that you think he is a creep, that you think he does things that you think are meant to harm you. There is nothing he could possibly do about your thinking. It's yours.

All that actually happened is that he suggested a prank, you told him "naa, not funny" and then he didn't do it. Sounds super harmless.

Things appearing on your desk are not intimidating. Unless it's a dead actual mouse or a bullet or something. Your coworkers computer mouse on your desk is just objectively not intimidating, regardless of whether he did it.

And you have zero proof or even hint that he did it at all. Things end up on my desk all the time. It's called cleaning staff. They are humans too and just as clumsy as the rest of us. Sometimes they sweep something off a desk and since they don't work there with all of you, they have no idea which desk that items actually belongs on. They will not leave it on the floor, they will put it on a desk, guessing where it might go, or maybe just picking the closest to where it ended up on the floor. Sometimes they pull all the chairs from the desks to clean beneath and then sometimes they put the chairs back in the wrong order. That happens. It's not a conspiracy or someone out to harm you. It's just other people living their normal lives around you.

What would you like as a solution to this issue?

What could he do to make it better?

Right now, it seems any interaction with him is something you see as negative.

However, even if he wanted to, he cannot just ignore you. Because that would be the one actionable, provable wrongdoing in the workplace. Discriminating against you.

I don't know what a solution might be. You may not want to be friends with him. You may not want them to listen to his suggestions. Tell him.

Maybe speak with others about it. If he makes a suggestion to you in private what to do and you think it would be harmful, talk to someone, a trusted colleague, your manager, the next group meeting, say "X suggested I do Y, do you guys think it's a good idea, should I do that?" If that is actually as you suspect a harmful suggestion to let you look bad, others will recognize. He will recognize that he cannot play games in the dark. But who knows. Maybe they step up and say "yeah, that's a great idea X had, let's do that". Maybe you are wrong.

If he doesn't stop, document it, then go to HR (or your manager) with documented wrongdoing and lots of witnesses.

Right now, there is nothing actionable at all. Just your "thinking", which neither he nor HR can do anything about, even if they wanted.