Guests usually expect privacy and flexibility, and an early knock on the door might feel intrusive rather than helpful.

This is advisable so long as the current tenant is informed, the unit is presentable & the necessary precautions are taken to minimize the intrusion & disturbance.

I even had one trigger just from heat (not humidity) in a vacant second floor unit in the middle of the summer.My solution has been to ensure those conditions are not created in these spaces, whether than means opening a window in the basement and using a box fan in the summer, or cracking open leeward windows on the second floor (in a manner to prevent water intrusion during heavy rain) on opposite ends of the building to create a bit of airflow in the summer.Lastly, I'll throw this out there in the category of "trust, but verify" - are you absolutely sure this is not tenant caused? 

Always budget for: roof/fascia/gutters, sewer laterals (get a camera scope every time), foundation/water intrusion, electrical panels, and aging HVAC.

BUT, I don't want to lose out on rent for 2-3 months while I complete the rest of the work which would include: 1) a new front deck (replacing an older worn structure with a couple of flaws that are long term concerns; but still functional and safe now)-there remains an alternate entrance for tenants to use during this time.2) bathroom rehabs (2); both small rehabs but ones that today I'm not ready to get into due to limited availability of my time this fall3) kitchen cabinets and counter, sink rehab:These are all things that as a standard LIF, would be fairly easy to work around (kitchen being the obvious most intrusive). 

The prevailing desire is to achieve the desired financial ends, in the most expedient means possible, within their risk appetite, as simply as possible with least impact and intrusion into their active daily lives. 

Water intrusion, leaks, or past flooding fall into this category because they can affect the property’s value, safety (e.g., mold),etc.  

Don't get the thin flimsy LVP, it tears when you drag appliances or furniture.Most basements I have seen have some sort of water intrusion.

One thing I can do is rewire the fan to turn on with the nathroom light switch, but I am not sure that would be enough.I'm looking for a solution that is the least intrusive/involved but still remedies the problem.I would rather not:-replace the window with one of a smaller size (this would involve disturbing the asbestos siding on the exterior, which is on the second floor, and doing a fair bit of demo to the inside wall)-raise the height of the drop ceiling (which would leave a 4 or 5 inch gap between the upper part of the wall panels and the drop ceiling grid; I plan on keeping the wall panels but replacing the tub surround panels)So...who's got a creative fix for this?

After that, we kept the units wide open until we got some significant rain to be sure there wasn't any additional moisture intrusion.