Question about a possible cat abscess?

My cat came from being gone for a week or two (this is normal for him) and when I picked him up he had a small squishy lump on his chest. The lump was maybe the size of a nickle. It was not warm and did not seem to bother him, there was no puncture wound or anything. I was waiting for my husband to get home to take him to the vet and get him checked out, but he took off again.

Today he came home (it's been about a week and a half) and I took him inside to examine him and instead of a squishy lump he had what looked like what may have been an abscess that was already scabbed over.

The area around the scab was clean and there was no redness or swelling. It was no longer full of pus, and the scab is about the size of my the pad of my pinkie finger.

It's possible that someone who also thinks he's their cat took him to the vet and had the abscess drained. However, if I need to take him to the vet I will. I would be concerned about giving him a double dose of antibiotics if someone else is giving them to him.

If the area stays clean and doesn't seem to bother him should I take him in next time he shows up?

Before you ask why he doesn't have a collar, it's because he always seems to slip them off somewhere; and the ones that can't be slipped off can be dangerous.