Tips on how to keep short-term rental guests from smoking in unit

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Hey BP -- I have a handful of short-term rentals and have them operating pretty smoothly. The only issue I seem to be running into is keeping people from smoking inside of the units (here and there). I'm looking for proactive ways to deter guests from doing this as I have several units within one building and want to avoid pissing off the longer-term tenant neighbors. I know you can require deposits, but I have found that this can deter guests from booking and when we did, our bookings declined. Does anyone use air alert monitors in the units or anything that is able to detect cigarette smoke? We are beginning to post "NO SMOKING" signs in the units with a note that says "$250 FINE" (knowing full well it's doubtful I could actually collect that). I know there is not a "solution" necessarily to this issue, but am asking those with experience for any tips/suggestions that could alleviate the problem to a degree. Thanks in advance!