When I started DMing that behavior was fresh new and exciting. A lot of "character play" has become cliché but stealing or even skimming from the group has become old hat quickly and now irritates me more than most stupid character play. Where previously I might have given the character "breaks" I don't anymore in fact I go out of my way to make sure other players pick up on it. Breaks as in if the PC was relatively clever with his description of his actions I'd cut him slack. How many DMs give other PCs rolls to wake up/ notice something is wrong/ realize that the rogue is bluffing if he says he wants to perform religious rites, alone, behind closed doors, unobserved, over the fallen body of a naga that might have a belly full of magical gear? And it was 2nd ed….
So spot/ bluff/ listen checks (as appropriate) all round for every occasion, and in the case of pick pocketing not just for the character getting pick pocketed.
Want to do your stuff while others are sleeping? Listen rolls, or even wisdom or intelligence rolls to have a notion that leaving the rogue on watch alone might not be such a good idea. It's one of the things your daddy or the adventuring school warned you about. The beauty is they all get to roll and at least one of them will make it. (While it hecks me off when they all get spot checks to spot my ninjas creeping up on them, and at least always one making it, I mind it a whole lot less during inter party checks.)
If through stealing/ skimming there is an item that is reasonably noticeable; i.e. a new magical sword or armor or similar, spot checks all round again. Even powerful amulets or rings get spot checks, or sense motive checks if the character is trying to hide them.
Karma starts coming into play; no more random dice rolls to see who gets attacked if there are two or more "equal" opportunities.
Of course I'd have a word with the player, quiet like, amend your ways, atone, confess to the group and return the gear/ money and I'll stop/ not start riding you. If he didn't listen either the group would sooner or later 'catch' him or he'd meet a sticky end because sooner or later he is in the wrong spot at the wrong time. I normally hate killing characters as a DM, but as the game demands a certain kill ratio to keep it interesting I hunt these wayward ones actively and they go nicely towards the quota of PCs I have to kill in a 3 month period to avoid the troops getting overconfident or lazy.