I have come up with a nice plothook to get the party to meet a recurring villain and I want to use an NPC that the party has previously worked with as the bait. The problem is that the party is now nowhere near a town or city and I don't know how they could be contacted realistically. Or more importantly: How could their location be found out?

The NPC is a non-magic user and I would prefer the message to be delivered as a letter instead of the spell Sending (As he has written the letter against his will and I just think it's more fun than a spoken message). My world is high magic so he could be using some magic service. But I need ideas. What are my options?