When Susan missed lunch, Lancelot was concerned but figured she must have been caught up with something elsewhere. Now that he hasn't seen her even once, all day, he has a bad feeling.

In truth, he's had something of a bad feeling since the appearance of the tower. That the tower seemed benign, that it offered supplies, did not particularly comfort him. The supplies seemed like a warning; in combination with the sudden appearance of the tower, the overall effect -- to his mind -- is a threat. So it's doubly unsettling that he hasn't seen Susan out and about, busy as she prefers to be.

It's nearly dinner time when he finally turns up at her door and knocks, calling softly, "Susan? Art thou at home?"