LOL! Sorry don't mean to laugh, but I gotta admire her aim!
My gelding , Walka, will do this to his mother's feed dish (I feed on the ground in large feed dishes, his is just outside the stall, hers is in the stall). I know he pees or poos in it for something to do. Just to darn smart for his own good and he wants me to see it to ! Mind you he has never done this in his feed dish! Maybe she's just plain bored and it's a game to her (?).
Be the kind of woman that when your feet hit the floor each morning the devil says, "Oh crap, she's up!".