Bob was going to retire from teaching in June, and the foreign language department was planning on presenting him with some luggage at his retirement dinner. He wasn't supposed to know about it, but someone let the cat out of the bag. At the dinner Bob acted surprised, even though someone had told him what he was getting before the official presentation.
All dictionaries list only one meaning—the suitcases and other bags that you take with you when traveling. Nothing surprising here obviously. But I'm not really sure how it's being used in the context of the situation described in the paragraph. Any idea?