Setup

King's Security Advisor contacts the PC's because of their known expertise in getting into difficult places. He wishes that they would see if they can break into the King's armoury and get out again. The Security Advisor (SA) has just updated all the locks, had new traps installed, and developed a new security routine for the guards. The SA believes if he thwarts the PC's attempts to get in then the armoury's safety should be well in hand.

The PC's break in successfully, or unsuccessfully, doesn't really matter. They get more glory for making it in and out safely and can supply priceless information on how to better secure the armoury. If they get caught, then the SA is shown to have done his job well, no face lost by the PC's because the SA is very good and the PC's get some good cash for the try. Win win situation - until.

A few days after the 'test' break-in before any new security can be put into place, the armoury is really broken into and many prized items are stolen. Who might be the likely suspects - obviously the PC's. If they made it once, they can make it again. If they failed the first time, then they learned a valuable lesson that wouldn't be repeated the second time. Perfect suspects, but they didn't do it...or did they?

1)Maybe the PC's did decide to break in. The SA was wrong in trusting them. No problem, the PC's just need to frame somebody and all the following could work for that as well.
2)The person that installed the traps is either unscrupulous or he was tortured to reveal the location of the traps and then disposed of. Once an investigation begins this man is found missing and leads investigators down that trail.
3)The SA has been planning the theft for years so he can gain the items. The security test by the PC's would further take him from the suspect list. Who would expect the SA?
4)Perhaps a few of the 'trusted' guards were involved. There would be quite a few people associated with the redesign and many guards would be used to guard those that worked on the project.
5)Fun way to release some powerful magic items as well. They can be used during the adventure but then must be returned to the King afterwards. A fun way to play with powerful magic, but then it doesn't upset the balance of the rest of the games.