I have no proof one way or the other sue. But my inner germaphobe says treat it as if it is NOT potable, just as you would any suspect source. I mean, how many toilets do you know that don't need to be fiddled with or fixed every now and then that require someone's hands in the tank. Can you be 100% sure those hands were clean? And like you said someone may have poured suspect water in there in the past. Who knows what else could happen to contaminate the source. Just my 2 cents. YMMV