She lives alone with no pets and we have no idea how it got there. 

When we do our PM WT's we are checking for tenant compliance with the lease (number of people in the house, pets, etc.) and also any obvious areas of concern (leaks, exterior holes in siding, etc.).

I would have to say that it's a pet smell.

Do you allow pets in apartments? 

I’m ISO of Insurance Providers that allow pets and don’t discriminate on dog breed.

Examples: I take a refundable pet deposit instead of a non-refundable one or a monthly fee, I currently pay the whole water bill without taking money from tenants because the duplex is watered together (I will eventually get a separate meter but the outlay is more than I want to pay right now) assuming it doesn't get out of control, paid for the new doorknobs on the bedrooms because the tenants requested locks despite the lead tenant saying they would pay for it, planning to replace the microwave hood simply because it's louder than it should be though it still works, etc.I'm probably losing out on a minimum of $100 monthly.

Newer block homes 1-1.5 miles outside of the central town still do well also, especially if kid and pet friendly. 

We have two dogs a pit bull and a chiweenie, but as a landlord we will have a no pet policy.

If you were a renter that didn't want to ever have to worry about a barking dog, had pet allergies, or had emotional trauma from bad experiences in the past with pets (especially dogs), wouldn't you be happiest in a no-pets unit, on a no-pets rental property?

Next we find out that she has 2 kittens in the house even though the lease specifies no pets.