Strange Abscess on My Cats Cheek

Question
A couple weeks ago, I noticed that my cat had a swollen left cheek. Just this past week, we finally took him into the doctor. They said it was just a random abscess and that something must have caused it. The vet drained it, gave him antibiotics, and let us take him home. Now, on Wednesday, they are doing a CT scan of his head. Could this mean that it's possibly cancer or something else that's serious? My cat is around 15-17 years old and he's an indoor cat. Any idea on what this might be?

Answer
Well I have no way of knowing because the timeline here is a bit confusing but most of all, some information is missing.

What happened between the draining of the abscess and the ct scan? I mean, why would they suddenly want to do a CT scan?

Did the abscess come back? Is there still a mass there? Are there other symptoms? Does he have a nasal discharge?

I would think that if a CT scan is warranted then there must be something serious going on. Most vets (honest ones anyway) are not going to do a CT scan of a cat's head for a normal abscess.

If the cat has a nasal discharge then a CT scan is going to tell them more than trying to stuff an ear speculum up their little nostrils.

So that would be a good reason to do one. Other than that I can't really add any more. Please let me know what they find on the scan.