Well, the behavior of a thief is just like Vivi's description:
No explanation, no farewell. Maybe they asked for the weapon and you gave it to them, maybe they promised to give it back, but in the end throughout some method they escaped. Basically that's theft, and it's unfair. If you want to see or have something, ask for it first, and talk with your party about it. Choose who gets the weapon >__>"
I think that it's unfair in the situation described, and that he clearly stole it. A lot of times, guilds have nothing to do with the person inside it, sometimes they just see someone and click recruit <___<" But thanks for the warning I guess.