Radiation detector or a leveling weight.

The thing that would concern me the most are the smoke detectors in the common areas (hallways and basement). they are supposed to be hardwired and connected. if one goes off all of them should go off. fortunately for you that is something easy to test.

I just added a window air conditioner to one apartment unit and the cost to increase the electric panel and wiring was $6,500 because the inspector made us install wired smoke detectors, GFI's in the bathroom and kitchen and we pretty much has to re-wire the apartment. 

Anything to do with safety make sure it is in order-railings, detectors etc.

Lack of gfci outlets where needed and other safety issues like missing smoke/carbon monoxide detectors. 

Seems like a metal detector might help find the lines/tanks.

When I’ve had inspections out to close permits they’ve always been sticklers for smoke and carbon monoxide detectors.

In speaking with other investors, seems it’s a normal thing to come through and check CO2, smoke detectors, etc.

Filters, refrigerator filters, batteries for smoke detectors and fan remotes, light bulbs, etc.Your list is pretty thorough though, I'd only add to make sure you're doing a semi-annual HVAC maintenance.

Some offices will hold tenants responsible for their items (like smoke detectors were there on move in but magically they are not there on the yearly, clearly a tenant issue) but generally speaking they hold the owner responsible no matter who actually caused the problem.