I hadn't even recognized the seeds the party rogue was planting throughout the campaign until they all came together in perhaps the most amazing heist I've ever played through. To be fair, I was quite thoroughly involved in a sub-plot involving a few other characters' backstories at the time, when I should have been involving the rogue more. Luckily, he made his own fun.
The party was working for a collector of many fine pieces of ancient art from a long dead elven empire. Statues, vases, various gem-laden trinkets, and so forth. Mundane, but beautiful and priceless. The party was more or less a glorified treasure-hunting team, combing ancient trap-laden ruins for plunder.
The rogue started his whole heist idea while I was describing the gallery of this collector. When they visited, they were searched at the door for anything suspicious; the rogue's lockpicks were confiscated, which maybe elicited a challenge, something for the rogue to prove, I don't know. Glass cases (all locked) containing all these priceless artifacts, including one black-bronze figurine of a gryphon that he was particularly interested in. I should have recognized from the outset that he was after his own personal Maltese Falcon.
Over the course of several adventures, the rogue built up a web of connections within the collector's employ, and buddied up to the collector himself. I thought nothing of it; it was nice to have a player so interested and invested in my campaign and the NPCs for once. I was forced to keep track of the little guys I made; the butlers, the cooks, the maids, the clerks. I fleshed out a small city because of the rogue's lust for details about everyone that worked the day-to-day for the collector's personal museum.
He had a keen interest in wood crafting, dumping many points in wood carving. I thought it a bit absurd, but he played it off as nothing more than a roleplaying reflection; a harmless, curious hobby of the character and nothing more. He went so far as to purchase masterwork tools for his trade, joining the local guild of wood crafters and discussing it with even more NPCs, peers in the 'hobby' of his character.
And then all at once, it was time for him to put his plan in action. During his downtime, he declared that he spent a lot of time visiting the collector's gallery, and carving replica figurines out of wood of the various artwork he had seen, roleplaying that each time he paid a little more careful detail to what he was making. And then he made the gryphon, and went so far as to decide he was going to make it a masterwork piece of art. I allowed this, naively. Every artist needs a masterpiece, after all.
The rogue needed a way into the estate, under cover of darkness, past the patrolling guards, while the party was distracted, without tripping the magical and mudane wards protecting the priceless art pieces inside their little glass boxes. To do this he:
- made friends with the trusted clerk of the gallery, became close drinking buddies with him
- waited until the day of a noble's gala that the collector had made public intention to attend
- 'requisitioned' the party's wand of dispel magic (much to the chagrin of the cleric for some reason), bought a scroll of alarm
- made sure the party was going to attend the gala, made sure to point out that, publicly, he would be attending in his finest clothes, and would be arriving late
- visited clerk friend in his office, informed him that after the gala, they ought to go drinking, and sleight-of-handed the lock on the window open without the clerk noticing
- convinced the women of the manor to get cozy with the household guards that night
Everything was written down on paper before the session had begun, in a series of prepared notes. I feel like he waited a few sessions until the timing was perfect (the gala) and then put his plan into motion.
At nine bells, in the blackness, with the manor all but empty and the guards snogging in the bushes with the chambermaids, the rogue made his way to the clerk's office. In through the window, out of the office into the gallery, a dispel magic to shut down the basic Alarm that had been set up; he made his way to the glass display case where the gryphon figurine sat. He spent a few minutes disabling a few traps, unlocking the case; he swapped the figurine for the wooden one, locked it all back up, reset the traps, cast Alarm and slipped out the window again. The entire process of swapping the figurines took about ten minutes, from start to finish.
He didn't say a word while doing this, merely passing the notes to me, and waiting for my response. I told him to roll dice as required, and then gave him a nod (of course I did, the little min-maxer couldn't fail at his tasks) and continued running the gala for the rest of the party members.
Eventually the rogue made it back to the party's apartment, put the bronze statue in his personal chest, changed clothing and attended the gala.
The party was none-the-wiser, and I was thoroughly impressed.
A few sessions later in the campaign, the rogue and the collector were together in private, when the rogue presented him with the bronze figurine. He made up some story to the collector describing how he recognized it in an acquaintance's home, and stole it back for him. The collector, obviously was flabbergasted (who wouldn't be), and said "There's no way! My figurine is in the case and hasn't moved!"
The truth comes out, the one in the box is a fake, the real one is replaced, and the rogue is given the replica (a masterwork piece of art in its own right) and a substantial, hefty reward for loyalty and friendship of the collector.
The party was floored OOCly when I explained to them what happened on the night of the gala. That was the happiest time I've ever DMed anything ever.
tl;dr - party rogue was awesome, pulled off an amazing heist, and didn't kill anyone in the process. Not even the party knew what happened.