No signs of fleas but scratches

My cat scratches here and there not a lot but I’m curious is if it’s normal or not. He’ll scratch and then stop until like maybe a few hours later and scratch once or twice again. I’ve checked everyday for fleas and can’t find anything and he’s had a flea collar on that I recently took off a couple of weeks ago because it’s month of use are up and I haven’t gotten around to buying a new one (it’s seresto brand if that is useful) he has had fleas before and I was able to find flea poop and stuff on him and he scratched way more than now but this was a whole year ago or so and we’ve already gotten him treated for the fleas, he contacted them from my MILs cat but she moved so we haven’t seen them since. I’m curious on to why he’s been scratching here and there could his skin be dry due to the cold weather? Or in general he has had dry skin when he had fleas and we bought him a lotion for it he also loves going into dusty areas so could that also be a reason? I try to clean off as much dust with wipes and stuff but not sure if everything completely gets off. He loves going behind our fish tank and on top of dusty areas for example this little area near the stairs that’s probably really dusty and he just sits up there and he’ll come down with dust on his head and stuff that I’ll wipe off with pet wipes