"I expect someone lives there in secret, only coming in and out at night, with a dark lantern. We shall probably discover a gang of desperate criminals and get a reward. It's all rot to say a house would be empty all those years unless there was some mystery."
"Daddy thought it must be the drains," said Polly.
"Pooh! Grown-ups are always thinking of uninteresting explanations," said Digory. Now that they were talking by daylight in the attic instead of by candlelight in the Smugglers' Cave it seemed much less likely that the empty house would be haunted.
I looked in the dictionary and found that drain = pipe, but I don't know what it really means here.