I did the tax collector murderer mission and it said she took the ring and when I closed the dialogue it was still in my inventory.
None of my mods have anything to do with that item and my saves were as clean as they could get.

It's hard to test this bug. It doesn't do it right after a load. I have to play a bit before I see it, and it can affect anything leaving my inventory other than dropping it.
I just tried reinstalling Morrowind with MCP 1.8 and am looking to see if that did anything.
Edit: Reinstalling didn't help. It just happened again to nelos onmar's note.
I have that part of the patch checked that supposed to fix it but it does not work.