When projects get hidden by various means, they dissapear from collections. 

This is very annyoing, because this also hides the button to remove that  item. And you do not know what it was and why it might be removed. The item is still there in the collection and increases the counter of items in the collection and takes a place in the grid.

Did I add  a scam or otherwise illicit project?  Or, hopefully more likely, did I add an item  where the dev made a new version as a new project and removed the old one?

I stumpled upon a "new" game and was very sure, that I only a few weeks ago put that on my backlog to view later. Turns out, that dev does indeed put up new versions as  projects and removes the old ones. I only know for sure, because of my browser history and when I try to access the page, I get an error message that I do not have access.

I get that some things are reversible, so there are reasons to keep the hidden item in a collection. But no info whatsoever is frustrating. Or is there any trick I am not aware of?

As for that dev, well, whith this technique he removed himself from by backlog. I think he even did it twice, because in my browser history I saw  three versions of his project.

There could be a placeholder with a link to the account at least. That would give you the ability to remove the item and check on the devs page for new projects.