Figure I'm the last person who should judge, my roommate still hasn't noticed I've been using her fancy olive oil for over a year. If my mug started refilling itself, I'd probably just assume I forgot I filled it again, my memory's that bad.
Wow, I used to be all "finders keepers" about weird stuff like this. But that olive oil story actually made me rethink it. If my mug was magic, telling my roommate is just basic damage control. Imagine it starts refilling with pickle juice or something one day, and I have to explain why I never mentioned it. Keeping it quiet is just asking for a way worse talk later.
Unlike @wren_thomas16's olive oil, a self-refilling cup is a clear glitch. Keeping it secret feels wrong, but I'd check it a few times before saying anything.