I've noticed it before, but passed it off as a mistake or hallucination, but seeing it now again, I have NO doubt - I have plugins disappearing on their own. 
Nothing was uninstalled, nothing at all was done, yet they disappeared.

Anybody else had this happen?
 

Did you check the administrator logs?

Yup, it only shows the latest "installed" entry. But it was definitely installed, since the marketplace let me install it without purchasing (meaning it was already purchased) and I don't have the files on my computer (meaning I didn't download it).
weird...

What is 'it' in this case?

It's the "Reply To Own Topics" plugin. But I've had it happen with other ones too (can't recall which).

Are you saying the plugins are just disappearing in the AdminCP under System > Plugins? If so, somehow the records are missing from the database and I couldn't begin to imagine how if someone isn't uninstalling those plugins.

I know, me too. That's why I started the thread, to see if someone else has encountered it or if I should go get my brain checked for multiple personalities.