Someone dies in a fire because you didn't provide smoke detectors.

The typica test includes a magnometer (super duper metal detector) as well as looking for signs of a tank (vent or fill pipes, cemented over holes in the foundation wall where the lines came in, etc).

My husband just went down in our tenants basement to replace the smoke detector and discovered water marks from a very obvious recent flooded basement.

I have a code enforcement inspection each year and the biggest issue is usually smoke detectors and any other safety violations.

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 !