Tenant stealing/sabotaging AC coolant ?

[Edward Tristram]

Thought we had seen it all.

I have a single adult tenant in an apartment complex who has been complaining for the last 4 months that the AC can't keep up (it is a simple single row 4 apartments with each unit having their own privacy fences so no one can see the AC from the road).  It is a relatively new AC and under warranty.  The technicians - we have worked with this company for a decade +, personal friends, and have never had any issues with getting problems solved -have been out there about 6+ times replacing items, charging it while waiting for parts, and unable to find the leak.  We went there today after a complaint over the weekend.  The last visit, we had replaced the locking caps and put new ones on them with specific marks to know if the caps were moved.    One of the caps was moved and there was 'oil' at the base of that cap, suspecting that the coolant was purposely leaked/stolen or whatever

The tenant has a family member in the AC business.

The question is what to do next.  Putting up a camera is really not an option without the tenant's knowledge.  There is an expectation of privacy there.  However, I don't know if the tenant is doing it or if someone in the area is stealing/inhaling/doing whatever with it (there is empty apartment building next door).  The tenant has access to the tool that would be required since a family member in the business.  

Do we try and put a camera up in the area that has no expectancy of privacy but is required to cross to get to the AC units for the building or do we notify the tenant and ask for permission to put up the camera (and freak out the tenant if not them, otherwise alert them we know there is stealing).   Having used video cameras on other properties I know they are not the best solution at night and it is easy to miss someone.