If you were burning fuel oil or coal or something like that stainless steel would be required but for natural gas you can use either material.

The closest to that is environmental coverage for Underground Tanks (normally gasoline or oil). 

I've heard the Zinser shellac based primer does a better job at locking in odors as opposed to Kilz...or make sure it is an oil based Kilz...Here's a link to a BP cat litter flip forum post from a few years back...half way down page 5 he talks about the smell removal process. https://www.biggerpockets.com/forums/88/topics/133059-cat-li...

Balloon" framing (1920's I believe).Radon.Plaster and lathe instead of sheet rock (pre 1950's) or plaster and wire mesh variations (50's) more solid than sheetrock but a real b*tch to repair or demo if you're remodeling.You've got Galvy plumbing but also need to look out for other kinds of outdated plumbing like clay sewer lines (pre-1930's, prone to collapsing, bellies, root intrusion), cast iron (1950's, often rusted and/or cracked by now), and PB (polybutylene, gray-colored water pipes, 1980's, prone to rapid degradation and subsequent bursting, mostly gone by now).Buried Oil Tanks-- Do you have those there?

It almost feels like I’m casting out a fishing line but no bites.

Today I got a call from the first floor tenant stating that there is a strong smell like dog/fish in her front porch.

And to build my portfolio up with some small fish then snowball into bigger.  

Perhaps, the heating system is old and inefficient oil heat and you modernize it to a high efficiency unit with a cheaper fuel like natural gas.

I understand I am not the biggest fish, seeking my first property and all, but should this concern me?

Quote from @Ashton Hines: Hi Devon, Curious what you think about the STR market around Lucas Oil.