Cat has scab of some sort

My cat got out of the house 2 days ago and we caught him soon after, and I just noticed a scab at the edge of his ribs. It's the size of a dime and totally caked over, but there's a bald patch around it which is how I saw it in the first place. Should I be concerned?

It's hard to tell just based on what you've written.  Is it bothering him?  If it's already scabbed over, it must have been just a scrape, if it only happened two days ago, at least that's my thinking. 

If it were me, I'd probably just keep a close eye on him to see if he starts acting differently, and if so, take him in to be seen.  You know, life if he appears feverish, lethargic, loses his appetite...that sort of thing.  Or if the wound gets bigger,that wouldn't be good.

I agree with Sally above and would just keep a close eye on the wound. It could be an abcess beginning from being scraped, punctured, etc. I would probably also check all over your cats body for any other possible dime sized lesions. It very well just could be a wound from something outdoors but just in case I would check through the fur.