Case 1:
Try to give it back, professional curtesy. Why would a stump grinder have a ladder? I would put it in my truck and try to catch up with him, or get the company name.
Case 2:
If it is a busy street (gonna get ran over) then I would take it, exspecially if it was a pro, not a homeowner saw. It would be likely it fell off the truck.
It is always better to try and give the stuff back. One day you will be workin, and they will drive by and see there junk, and take yours.
If you try to give it back, and honestly cant, then you just got a ladder and a saw!!
Why not tell what you would do too.