The rightfull owner could send you a signed message, BUT if i would be the owner, i would have no idea who to contact. I dont think alot of people are reading your thread. It could be any wallet... maybe a cold storage or something. You could wait years before someone show up. And even if the one realizes, his coins are gone, how could he know you took them.
I think you are at a point where you have to decide if you want to stop this, because you have proven it works, or if you want to continue, but this could be called theft.
Which way you will go?