It's also probably a pretty rare thing. If the other wizard is less powerful than you, you can probably get nicer stuff on your own without stooping to petty theft. If the other wizard is more powerful than you, it's borderline suicidal to give them a direct sympathetic line to you by having their stuff. In terms of magical stuff, most of it is specialized or personally attuned in some way and won't be as useful to anyone other than the wizard who owns it. Essentially, it's absolutely not worth the trouble for wizards to commit acts of petty theft against one another.
For acts of grand theft, when it's worth the while of high-level wizards to cast divinations into the matter, it's nearly impossible to get away with it anyway.
The only time it's ever gonna happen is if one wizard has something that's
a) useful or valuable to anyone else
b) that is sufficiently unique or difficult to obtain that another wizard would rather deal with an incredibly dangerous individual - with access to magical divinations and traps - than pursue any other avenue of procurement
c) the wizard in question cannot or will not simply bargain or negotiate a trade, purchase, or borrowing of the item
d) the thief is confident that scrutiny from high-ranking members of the Council - people with centuries of experience and broad and powerful abilities to gather information - won't put him squarely in the Wardens' crosshairs.
That's a lot of ifs. All it takes is one victim running to the Merlin and getting your theft scryed to, at best, give you a reputation as a liar and thief before an audience of immortal peers with every reason to hold grudges, or at worst getting you in hot water with a law enforcement organization known for favoring capital punishment.
Stealing from other kinds of supernaturals has most of the same problems.