If they have any sense they'll not try to find out what's on it and send it straight to whatever electronics recycling is available.
Sticking a USB device of unknown provenance into your computer is just asking for trouble. (When you think about it, we even take a risk every time we buy one.)
Sure, you know it's harmless, but they don't know that, even if you tell them. Who are you? You're just someone who used to live in their house. As far as they know, you might be a freak who gets a kick out of leaving dodgy devices around for people to find.