Thanks very much for the comments, Katluvr, Moonshadow and Zapkid! Yes, Mr Payne gets around. He can also be found in the Swap Shop, where his biography includes this line:
His raison d'etre is to help other Sims.
Which may give us an idea what he's up to here! In any case, the story will be finishing on Saturday - just 3 more installments, so it won't be long before all shall be revealed.