“ A casual remark by one of the party about something met on a previous adventure leads to a summons to and grilling by the Mages' Guild (or equivalent), probably resulting in an expedition back to that area to recover the things for study by the Guild. The party's presence will be 'requested' for this expedition.”
“ He's a non-descript man, with his pushcart. On it he sells nothing more exotic than jars of sun-dried tomatoes in oil and pickled vegetables. But he's always out there, in the courtyard of the great Guild of Wizards, in most weathers, and he'll have a kind word for you, and a jar.”