Wood is very porous and will hold that odor for a long, long time...

I am about to start an Air BnB and am concerned about smokers, pets and general odor elimination between guests.

Have you ever had a tenant argue with you about an odor?

So purchasing that asset and rehabbing it as multi-family can bring grumblings from the city.  

You will do a better job if you are alone and follow a checklist to ensure nothing is missed.You'll also find that a lot of odors can be masked right when they move out.

Inevitably, animals leave a trace (odor, scratching, and so on).

When this happened in one of my buildings, I went into the complaining neighbor's unit and smelled the odor myself. 

Perhaps I am too small fry (looking for sub 100K loans), don't make enough money, have a personal odor, I don't know and they won't tell me!

I just googled "what causes my water to smell like sulfur" and had a "water heater odor killer" pop up.

I would even be likely to turn a blind eye to the dog being in occasionally (since it was my mistake to allow it in the first place)  if I hadn't had multiple complaints about the strong pet odor.