Cat pee smell when I come home??

I don't know what to tag this but I'm looking for some advice!

This might've started a couple months ago now but every time I walk through my door, I smell cat pee. I have checked every where in my house and can not find where it is coming from. I have noticed that it smells the strongest around the vents and most noticeable when the heat is running.

I have two cats who have never peed in the house. I have gotten down on the ground and smelled just about everywhere. I thought maybe they peed in the basement because they get locked in there every once in a while but I haven't found anything. I even got a blue light to check for urine spots!!

I live in a twin home and can sometimes smell when my neighbor is cooking dinner. I'm wondering if it's possible it's coming from their home? I'm at a loss because I literally cannot find the source and I'm usually really good at finding cat pee. I'm also wondering if anyone has ever experienced something similar to this.

Thank you in advance for any insight!!