A gaslight sounds dangerous in this situation.

Its all about confidence really.

“I don’t smell anything. Are you feeling okay? Maybe you are having a stroke.”

“Have you ever noticed how the drains round here constantly get blocked and release sewer smells onto the street”?

“Ooh, there’s that weird smell again… I wonder where that came from?”

“That factory down the road needs to stop venting that stuff into the air”

“Eew, I hate that smell when the air conditioning’s filter breaks”

“*sniff* I guess the farmers are spraying fertiliser again”

“That’s odd - does this hammer smell like farts to you?”