The party has just cleaned out some ex-mage's compound. In the scenario I was working with, the party had found a virgin ring of spell- storing and some matched jewelry, but just about any similar high-power magic would be useful as a set-up.
The party is resting from their endeavors when a well-dressed person comes to find them at their current quarters. He is an emissary from a high-level noble of a nearby country. He asks if the party is the one that cleared out the mage's quarters. If the party denies it, he produces proof. After the identification, the emissary asks if they have the magic item. He explains that the item belongs to his master, it was commissioned and paid for. He demands the item and offers little or no (DM's choice as necessary to provoke the party) reward. When the party refuses the emissary explains that by the laws of the country he comes from the item belongs to his master and they must return it to him.
If the party still refuses, the emmisary declares them outlaw (something most countries ignore) from his country.
Whenever life is getting boring after that, send an assassin or two or maybe thieves to steal the desired item after the party. If the party tries to go after the noble they will have the difficulty of manuvering in a country where they are outlaws. The whole setup provides a good hook for several other plots and can be used to cause havok wherever the PC's go.