Six month old Rott has a small hole in his chest.?

This "hole" is an indention that looks like someone just neatly removed the tissue and it has healed. Its about the size of a pencil tip. He doesn't notice it, but I found it today. No redness, swelling, discharge (so far). Definitely not a BB pellet, he lives inside 24-7 and has only been outside with a leash. I thought it was a nipple to start with, but then realized that it didn't stick out, and actually was a hole. He has a vet appointment tomorrow, but I'm going to go crazy between now and then. Any thoughts would be greatly appreciated.