Over the centuries, the thieves, assassins and secret agents who operate within the underworld have come up with many ingenious devices for stealing and murdering, many of which are disguised well enough to be carried in plain sight.
The player characters, experienced and somewhat well known, hear rumors and travelers' tales about a distant area being overrun by dragons (or other terrifying monstrosities). The locals have sent them a message, begging for heroic aid.
When they investigate, they discover that nothing of the sort is going on. It turns out that a group of thieves wanted them out of the way so that they could rob them (or someone who would normally receive their protection).