Feliway Huffing

Looking for any behaviorists (or people familiar with behavior) to chime in.

I've noticed both of my younger females (~3y FS DLHs) hanging out by the Feliway diffuser from time to time and sniff it for several minutes straight. It doesn't seem to be making anything better or worse behavior wise for either of them, although I do worry about the Shitten's asthma and her straight up sitting there huffing the diffuser. It's only these two that we see interact directly with the diffuser ever, even though it's at cat level. Is this somewhat normal behavior?

I'm also wondering if this is a thing that's expected that I should let owners know they might do when we recommend diffusers at work.

I started using the diffuser in the house for the Shitten's inappropriate urination and redirected aggression towards the older female cats (which it's done a lot of good for) about 9 months ago. My mom just recently pointed out the excessive sniffing within the past month, which has made me notice it more, but I might just need to take my own advice and stop staring at the cats.

Cat stats:

Calli-co Shitten, almost 3y FS DLH, ~6kg; hx: feline asthma; anxiety; FUO as a kittenCritter, almost 3y FS DLH, ~5.5kg; hx: suspect permethrin toxicity as a kitten causing mild neuro changes; deaf; blood donor