What does Mrs. Corney catch Mr. Bumble doing when she returns?

She caught him going through her drawers.

"Having listened at the keyhole, to assure himself that nobody was approaching the chamber, Mr. Bumble, beginning at the bottom, proceeded to make himself acquainted with the contents of the three long drawers: which, being filled with various garments of good fashion and texture, carefully preserved between two layers of old newspapers, speckled with dried lavender: seemed to yield him exceeding satisfaction. Arriving, in course of time, at the right-hand corner drawer (in which was the key), and beholding therein a small padlocked box, which, being shaken, gave forth a pleasant sound, as of the chinking of coin, Mr. Bumble returned with a stately walk to the fireplace; and, resuming his old attitude, said, with a grave and determined air, "I'll do it!" He followed up this remarkable declaration, by shaking his head in a waggish manner for ten minutes, as though he were remonstrating with himself for being such a pleasant dog; and then, he took a view of his legs in profile, with much seeming pleasure and interest.

He was still placidly engaged in this latter survey, when Mrs. Corney, hurrying into the room, threw herself, in a breathless state, on a chair by the fireside, and covering her eyes with one hand, placed the other over her heart, and gasped for breath."