Hi there.
I have eight in/outdoor cats, and have managed a few flea outbreaks in the past. I would say your flea problem seems a way short of an outbreak, so don't panic. I assume your cats go outside? OK, let's work on that premise.
My cats can be flea free but still go outside and pick up unwelcome visitors from the outside, these may be the one's you're finding on your cats. They will be killed off by the topical flea treatment you are using so long as it's still effective.
Indorex household spray works to kill any fleas, larvae and eggs that have taken up residence, and breaks the flea's lifecycle for 12 months, so you only need to treat the house once a year.
I use Advantage every six weeks or so, which is now available without prescription from Animed (on-line, free postage) so long as you register your cat(s) on their site so their resident vets can monitor what's being bought.
I swapped from Frontline when it ceased being effective for us last year. Indorex is also available on the Animed site, or from VetUK (you don't need to be a vet to use this site, by the way!)
I don't have carpets either but still treat the hot spots in the house (cat beds, trees and under radiators for example).
Last tip, when you spray, leave it overnight then hoover up any dead bodies, then fling the bag in the outside bin in case there's any live ones in there.