Spray, or a metal device, or is there something new on the market you can suggest ?

:Look at many metrics and actually thoroughly learn them:IRRgood for evaluating dissimilar projects but doesn't given magnitudes or timing of cashflowsCash on Cash ROIIf you're putting down 300k to pay in full and then only cash flow $100 bucks a month, although it's cashflowing, I'd barely say that's something to write home aboutBreak even YearYear when profits turn positive and stay positive......This whole discussion may also have an undertone that one should, although it may be difficult at first to setup, geographically diversify to mitigate risk in any one market.  

Their flat rate monthly service covers all materials and labor, wear and tear, tenant turnovers, repairs and replacement, preventative and comprehensive maintenance and home improvements,  Unlike home warranties, they don't charge any deductibles for repairs.Maintenance requests are reported by the tenant using their mobile video app or detected by IoT sensors from connected devices, and sent to the landlord or property manager for approval.

And while your in-house team could certainly incorporate these devices into a routine inspection/maintenance program, that will come at a cost. 

During an inspection by the city, it was noted that the system lacks a backflow prevention device, which is not up to city code.3.

In most states, an undisclosed audio or video device placed in the interior (non-common) of a private residence would be illegal and could also result in a civil lawsuit against the landlord. 

I have a BS and MS in Engineering and I design Class III medical devices for a living.   

I didn't try using it that way, but the device does allow for that.

We found a device for smoking pot. 

Tenant erected a 12 x 12 outdoor extendable living accommodation shed in the backyard without any permission installed permanent tiles, ran wiring in the shed for lighting, fans, for electrical devices and also for air conditioning.