Ah, so it's not pee? Bummer. Been there. Seconding startingsmall's comment. Have her checked by the vet.
Our problem kitty had IBD or possibly GI lymphoma, exacerbated by stress, and she associated her litter box with pain after a few bad episodes and started to avoid it when she felt ill. It was intermittent and hard to solve.
If it is not medical, another possibility is that your cat's normal box is not clean enough (plastic boxes do start to retain too much odor to clean after a while and need to be completely replaced) or she doesn't really like the litter you've been using.
Yet another possibility is that the normal box is located in an occasionally stressful area so she seeks out a more private location that smells appropriate (thanks to previous dog) when her normal litter box is unavailable. Think back to her accidents, was there an unusual event, like a guest in her space or someone cleaning her space with a vacuum or anything? If so, you may need to consider providing her with a second box in a separate acceptable space with more privacy for those events.
If all else fails, my suggestion is to accept it and put a litter box in that spot but clean it a bit less frequently to still encourage use of box #1.
Good luck!