And two of the units don't have carbon monoxide or smoke detectors.

What types of routine or emergency maintenance will it need (water leaks / exterminator / smoke & CO2 detector maintenance / air filters / gutters / outside maintenance / paint / flooring / refresh between tenants, etc.).

Does anyone go over their properties with a metal detector?

I make sure to check all smoke detectors, sinks, toilets, electric, etc.

It has a closet, smoke detector, overhead light, multiple electrical receptacles, and two sets of windows. 

Smoke detectors , working toilet working sink , good working outlets , functional doorknobs no trip hazards .. simple stuff !

I have no clue on insulation, windows, fire alarm, CO detector, fire retardant door thickness.

And fire, smoke and carbon monoxide detectors. 

I do most investing outside the city in delaware county, but i would think there is an inspection they do yearly as a run though to check for smoke detectors and things of the like. ( at least that what they do in Delco ) when they are due, you'll know for sure

In May 2017 the boiler had a backdraft and the carbon monoxide detector when off in the utility room when off so they evacuated the home and called the fire department.