Do you think it is ok to secretly sniff a female friend's laundry?
That is weird, why do you want to sniff her laundry?
I'm a secret fan of her mint scent from her laundry
Sounds like you're attracted to her. Why not ask her out, or ask her what kind of detergent she uses? If she finds out you sniffed her laundry behind her back, she might call the the police at worst, or at least think you're a total creep.
I don't want to ask her out or tell her. It's no big deal since no one will find out.
One day she may very well find out and it could ruin your friendship with her. Just tell her she smells nice and ask what she uses to freshen up with.