I am not completely sure how this one ended, but we had a similar situation from the other side. We were visiting a small city in eastern Canada, back when cell phones were flip phones without any security. Walking to dinner we found a phone sitting on a park bench. We just called the last number that the phone had called and left a message telling them where to find the lost phone.
We passed by the same park bench on the way back from dinner and the phone was gone. I like to think that the right person found it, but we never found out.