If the levels legally require remediation you will pay for the testing and if they do not the bill will go to the tenant.You can also take the alternate approach of going into the attic with bleach or a solution of distilled vinegar and scrubbing the area that the tenant reported had mold.

Scrub carpet by hand with sponge in Odeban carpet cleaner ($10 @ HD) & water mix.

We had to renovate a unit that our tenant smoked in for years, and it took several weeks of scrubbing everything with TSP, then 2 coats of KILZ original, then paint.  

If you can find evidence, or have an idea of where the mouse has been and the paths it tends to travel, disturb all around those areas daily (especially after dark), by vacuuming, mopping, scrubbing, moving items, etc.

Airing it out, scrubbing floors and walls, painting - none of it succeeded with clearing the stench.  

We literally discard any leads that owe too much on mortgage ( I know there is something called creative investing where you can still extract a deal out of such leads... but why would we.... there are soo many we can cherry pick).So we scrub all the leads that are "no good".

Scrub it really well and see how things look. 

Scrub the walls with a mildew/mold killing cleaner, same with the carpet. 

I kind of wondered if something like that may have happened - where they wore away the coating by scrubbing a stain. 

Even things like paper towels they used for cleaning, an empty trash can, etc.Did not empty and wipe out all cupboards and cabinetsDid not clean appliances - this means taking shelves out of your fridge and washing them, scrubbing the stove top clean, cleaning the oven, cleaning inside the microwave, etc.