Honour Among Thieves
They say there is no honour among thieves. Those who put that to the test usually find themselves serving as an example to others, and as a helpful guide to the prevailing winds.
The Art of Keeping Your Mouth Shut
The theory behind this is fairly simple, but one that is worth re-iterating merely because the importance of it cannot be stressed enough:
If you are ever asked if you stole an item, or if another thief has stolen an item, unless you have personally stolen the item and wish to claim credit, do not reply in any affirmative or negative way.
While the affirmative may be obvious (that can get you smashed, or at least lead to an awkward encounter) saying that you did not steal something is surely a good thing, right?
Well, no. The theory runs that Player A asks Thief B if he stole an item from them. Thief B says they did not. Player A sees that only two Thief playerkillers are online, so proceeds to take his frustration out on Thief C.
Thief C may have not actually stolen that item, it might have not even been stolen at all, but someone, somewhere, will always take the blame.
So by refusing to answer, you make every playerkiller thief complicit in the crime. There is strength in unity. So while the player may be looking to smash someone for their item going missing, they have a more difficult choice on their hands.
So in summary: Don't stitch someone up, even involuntarily.