My property manager figured out that he runs a credit and backgound check, criminal backgound and checks with previous landlords and still notices they check out on all fronts and if that is a green light then he checks one more thing that is detecting some bad ones.  

On the repair side, I don't offer fridges with water dispensers and I have vinyl under pretty much every sink amd dishwasher that runs up the back wall a bit to prevent not detecting slow leaks that turn into huge repair bills.My people know all non-emergency repairs are taken care of on Tuesdays or Thursdays (depending on which complex) if we can't address it by text.

Which caused late detection of leak due to her negligence.

I don’t believe Co2 detectors will detect natural gas.

He comes within 2 days and with extensive tests find there are leaks from the shared pipes throughout the HOA and our place has detectable mold. 

And you can't play detective either. 

Leave an inch uncaulked in the very back so leaks can be detected sooner than later. 

All the time you take turning into a detective to see if the "reason" they are a felon is acceptable. 

Again whatever you feel comfortable with in terms of escalation here.No personal experience with this device and not sponsored, but I've looked into FreshAir Sensor previously for my Airbnb, and it is a simple plug and play device that detects cigarette smoke.

We have to be our own detectives sometimes and this is for sure a great way to find out some information.