Mr. and Mrs. Pigeoncote have always been reluctant to invite cousin Wilfrid to stay with them, as he's an incorrigible kleptomaniac. This means that he has an uncontrollable impulse to steal things. Ordinarily, this would be reason enough not to invite him, but as the Pigeoncotes have just celebrated their 25th wedding anniversary and have lots of expensive gifts lying around the house, they're even more reluctant to take a chance and let Wilfrid stay.
Nevertheless, against their better judgment, the Pigeoncotes decide to invite Wilfrid over after all. Their snobbishness has got the better of them, as they wrongly believe Wilfrid to have inherited a baronetcy. But baron or not, they resolve to keep an eye on him, especially around those seven silver jugs they received as an anniversary present.
When one of the jugs goes missing, suspicion inevitably falls on Wilfrid, but when he says that the silver jug in his suitcase was his gift to the Pigeoncotes on their anniversary, Mrs. Pigeoncote is rather embarrassed to say the least. When her husband goes off to bring her some smelling salts, Mrs. Pigeoncote tells Wilfrid—falsely—that her husband is actually a kleptomaniac. This is her way of providing a rationale for searching Wilfrid's belongings, thus avoiding embarrassment.