find my mac found

In this article If your Mac is lost or stolen - Apple Support there's a section How to unlock my Mac.

Talk to the person that owns it, do the procedure while talking to her and make sure she can use it again after you unlock it.

Also see this What to do before you sell, give away, or trade in your Mac - Apple Support

Oh I see, you found a lost/stolen Mac.

Well, the right thing to do is to report it to the cops, but maybe you can go first to an Apple Store or Authorized Service Provider and ask them help to find the owner. Maybe they can help with it, plus you're not asking them to unlock the Mac but only to find the person and maybe set a meeting with her at the Apple Store so you can be sure of the ownership and everything can be done safely.