I could see it being something like "locate mark" where the thief calls upon his various contacts all over Thera in order to divine the location of something.

Definitely don't think it should bypass nolocate, or allow the thief to locate items whose level is higher than his own.

Should definitely cost money, have a longish timer (like loacate mark) and should be a skill you have to spend thief points on.

Also, might make sense to restrict it to sufficiently rare items, since those are the one a thief's network of spies are going to have "noticed" on someone. It's not realistic to expect a spy network to keep records of everyone they see with charred leather bracers, but it might make sense for them to make note of someone carrying humansunder.

Also, that mystic skill that prevents people from seeing what you're wearing should make you immune to "thief locate".