If it looks, smells, sounds, and acts like a duck...

The house smells bad, and is very messy.

I am looking at a property that smells of cat pee.

I own 3 SFR's in San Francisco including one that is zoned as a 2-unit and one that looks, smells and feels like a 2-unit but technically is not (and is connected by a staircase to the "other floor").

It smells of gas in one in the kitchens.

The smell and damage to walls?

We just had some prospective tenants turn in applications and they said “non-smokers” on the application, but I noticed they smelled strongly of smoke and even their applications (the paper!)

Wired smoke detectors are usually all interconnected, so if one smells smoke, the horns on all of them go off, but the signal that makes this happen is not standardized among manufacturers.  

The sludge comming out of the lines looked like tar and smelled much worse .

What unites all of them is a phenomenal economic growth phase (for midwest standards anyway) and haveing a large body of fresh water just down on the beach.