While a metallic footprint consistent with a large metallic object present, the absence of a fill pipe or vent pipe necessitates the need to excavate the area with shovels.

However, they left some rust stains in the corner of the bathtub (what looks like from a metal can they had there), I've tried CLR, Lime away, bleach, etc. and I cannot get this to come out. 

Even if I met all those standards, I will be discouraged to rent it because I may think somewhere between the lines, I don't qualify.

Any metal etching primer? 

Met all of the tenants when I was first looking at the place with my realtor and they seemed like they'd be tenants that I'd want to keep.

For example, a metal door, I will use a semi-gloss oil based paint.

They are a fairly cheap to build metal building with minimal land costs meaning they can easily jump locations and from what I have seen they have absolutely no problem with moving over one lot and rebuilding at the end of the lease. 

I purchased a 3family home in Philly (it already had a system I think) big red metal circle and fire alarms on all hallway floors. 

I have met all the tenants in the house except for one lady because she just moved in last month.

I have even ran metal conduit on the outside of the walls to save damaging the walls, which was still a couple grand.