Jonathan, we had a similar problem with guests bringing “unregistered” guests to our large home on 10 acres. Like you, we didn’t want to price ourselves out of the market and we didn’t want to block 1 night stays, as this is our only income now that we are retired, so we added cameras outdoors and changed our policy to allow for 6 guests at the regular rate, to keep the rate low, and any additional persons onsite for any reason or length of time we charge $35 per person, per day and they must be registered 2 days prior, or they are not allowed to come.
We also require a copy of their photo ID at check in to be sent or shown. As these are listed under the rules section, if they don’t comply, we ask them to leave. This solved our problem for the most part.
You could put a limit of 2 guests, or 4, or whatever you prefer, and must be registered 2 days prior and MUST show / text valid photo ID for all guests 18+. That should stop the criminal element. We have had people book for other people (who are banned or underage) so you should check it against the listing photo. We have also had guests leave illegal drugs in drawers, where a child could reach, and where the housekeeping staff did not find (I did thankfully) so we added “no guests allowed on-site with a criminal record”. It’s harsh but necessary for the safety of all guests.
It could be they are going out to smoke cigarettes- our European guests typically step out to smoke on the patio every 30 min for a few minutes.
THANKFULLY 99% of all guests are amazing wonderful human beings!