So glad you stopped to help him.
He may be marking out of desperation, trying just to claim some part of the world for himself. He may stop after he feels a bit more secure. My 13 year old neutered male started marking furiously when I added 3 girls to our pack. I think he felt his space was invaded by aliens and was trying to let them know they were in 'his house'!
The microchipchip issue: All newer scanners will pick up a chip, but can only read the info of one from the same manufacturer. Some will id a chip, and can tell you the type (what company). Most shelters have universal scanners that can read all chips, regardless of who made them. Sometimes chips can migrate, so be sure the vet does a full body scan if one is not detected in the neck/shoulder region. My vet has both a universal and the one from the company they use (HomeAgain). Some companies will allow you to register or at least list chips from different companies too, so if you purchase a HomeAgain chip but your other pet has a 24petwatch, you can list them both with each company.
I would talk to the owners should they come forward... get a sense of what kind of owners they are... maybe they have given up hope of finding him and moved on, and may even want you to keep him or find him a home. Peek a Boo was a runner. He ended up at the Humane Society twice, and I contacted the last owner and she said the 2nd time they decided just to leave him there!
Obviously NOT Yorkie people!
If I can help you in any way, just let me know...
