I'd say just drop the bomb on her. Back before I was together with my Girlfriend I used to hide it from her then one day she got into my room before I could hide everything. (cue slow motion running and yelling NOOOOO!) Anyway she just laughed about it, she was probably more thrown off by the mountain of dirty clothes.
So I think telling her upfront is a lot less stressful in the long run. Plus if she doesn't like it you can justify your collection with "Well does your spoon transform into 3 different modes?"
Yeah that will put her in her place
Good advice buffalobob77! Other than that Powered Convoy, I don't think you have nothing to worry about with your current gf.