If a player has his character steal from his allies, then he deserves the same reprisal that any other thug would get if the PCs caught him stealing form them. In most games, that at the very least should mean getting the crap kicked out of him and having most of his stuff taken away, with the promise of death if he ever bothers them again. Stealing from your friends is pretty despicable in real life. In worlds where capital punishment is fair punishment for theft, it would be pretty easy to justify killing someone for tryign to steal from you...or cutting off a hand, etc...
Bluff is basically useless in PC interaction...the PCs get to more or less rationalize whatever action they want. Usually by obscuring the line between player and character knowlege. If the player knows the PC took something, then suddenly he will find an excuse to say...cast detect magic on his friend. Then suddenly the party will want to do a total 'all gear on the table' inventory, etc...Someone will always go into rooms with the rogue and look over his shoulder into every opened chest etc...I've seen it happen.
The PCs casting detect magic on the rogue doesen't really prove anything. It doesen't even prove the rogue has any magic items at all. He might be a sorcerer/rogue casting magic spells on his gear. Or he could have a potion in a belt pouch, or any other number of things. The PCs do not exist in a vacuum where every PC is intimately aware of every action every other PC makes. They have down time where they are on their own. Unless they catch him with his 'hand in the cookie jar' they can't really prove anything. Zone of Truth or divination may help, or maybe not. But ultimately the PCs will do what their players want them to do. Speaking as a player who has had player conflict ruin campaigns and break up gaming groups, here is my suggestion. Don't let PCs screw with other PCs. Enforce the unwritten rule that other PCs are off limits.