Slightly modified Permanencied teleportation circle at the top of however many flights you want to actually have? Circular staircase, so the party can't maintain line of sight very far. Activates at the end of the round anyone steps on it and takes them back down to the bottom...with any tell-tale symbols painted over or otherwise concealed. You should allow a save to notice it. Or relevant skill checks, stonecunning, etc.
As far as a spell to -actually- make a staircase endless....sounds Epic to me. Though the idea is simple, the magic seems big.