If you've ever had to deal with these pests you know how destructive they can be. What you may not know is that a piano is a great and safe place for them to hide.
Unfortunately it's not so great on your instrument. If left undisturbed for any length of time you will soon be over run and your once fine instrument might just need hundreds or thousands of dollars in repairs.
How do you know if you've had mice in your piano?
1. Smell - if you have ever had mice there is a distinct smell.
2. Droppings/feces - open the top of the piano and visually inspect the top of the keys and open the bottom panel to inspect. The droppings are generally small pellets about 1/16 of an inch around and less than 1/4 in length.
3. Staining of the key stick - as the mice traverse around the interior of the piano they will leave subtle staining. Looks for a "path". These are darker areas where the body of the mouse has rubbed repeatedly against the keys.
4. Gnawing evidence - look at the key stick and see if the edges are crisp or if they appear to have damage. The pictures show subtle rounding of the keystick. Additionally if you were to manually move the hammer forward the bridle strap should pull on the whippen lifting it up. Mice often eat through the bridle strap first. This material will be used in the nest.
You have 2 options here -
1. Remove the keys from the piano and vacuum the debris out of the piano.
a. Mark the keys as you remove them so that if they get out of order you can replace them in the proper places.
b. Then repair any damage.
I suggest wiping down all keys with a mild beach solution as well as under the keys being careful to not contact the keypins.
2. Have a qualified technician clean up the mess. Some technicians charge an exorbitant fee to clean up after rodents. I've seen quotes for over $500.
There is some possibility of contracting certain diseases during this cleanup. I always recommend wearing rubber gloves and a dust mask or respirator when cleaning and vacuuming a piano. I would suggest you take this precaution at a minimum, and if your are immuno-compromised I would suggest having the work done for you rather than tackle it yourself.
If caught early, mice will do very little damage, so at a minimum have your piano tuned once per year and it will be opened up. Most technicians who are paying attention will notice the evidence of mice and notify you.
The two pictures below show evidence of gnawing. Notice the top edges of the keys at the 'V' gap are rounded