Who spotted Mr. Tom as he tried to slip in unnoticed?

From the text:

He slipped quietly to the back of the hall, catching sight of Nancy and Dr. Little, and acknowledged their presence with a slight gesture of his hand. He attempted to stand inconspicuously in a corner but it was useless, for most of the villagers nudged one another and turned to stare in his direction. Tom, as Zach said, kept himself to himself.