My store has been plagued with fruit flies for many many years. There have been at least a dozen times where I clean something that hasn't been cleaned in years (or ever...) and I think "Yeah! I've got it! This is where they were living!" and then the flies get better for a couple days and find somewhere new to live. It's insane. Pest vendors are absolutely worthless. They tell me to clean something. I clean it and there is zero change. The only things that help are cleaning the drains daily and leaving out apple cider vinegar and soap traps, but those just kill the adults and don't fix the underlying problem. We could have definitely gotten an NNC every summer for the last half dozen years, and even some winters! The DMs I've had just don't seem to be looking for the flies very hard. They'll notice when it's bad, but that's it.