As My Guitar Gently. ... Pee's?

After putting the girls down last night, I picked up my guitar for a little "me" time.  I was particularly perturbed to hear an awkward rattling noise coming from the inside as I lifted the guitar up, which I first assumed was part of my pickup.  As I moved the guitar around and strained to see what was inside, I spotted a bright orange "p".  Now, none of our magnetic fridge letters made it to the new house. ... so I'm kind of shocked that the letter "p" has myseriously shown up. ... and somehow made it's home inside my instrument.  Now the question is. ... who p'd in my guitar?!