Odd suggestion, but do you still have their contact info? Could you ask them? 😄

You could also send them a small thank you gift and ask them with that, so not to make it seem like you're accusing them of anything

It's a reasonable request, you could say that you need to keep your phone secure for work, and while it was great that the stranger was able to get it to you, you're following up on if there is some bug you need to look into

Unrelated, it might be good to set up a "If lost, call ____" type message. If you don't have another number, email also works.

Is your pin simple? If you hold your phone up to the light can you see the smudge marks where your pin usually goes?

Do you have people set up as emergency contacts via the lock screen?

It might have been the fingerprint sensor. They can be fooled. Mine occasionally thinks the inside of my trouser pocket looks just like my finger.