The Spy Game?

My son has a play date today with a girl from his class. They created a "Spy Club" which seems to consist, in part, of hiding "spy gear" around the house. I'm not sure what this spy gear is though.

Then they sneak (very loudly) around hiding behind the sofa watching me type on the computer until they burst out laughing and run away. This has already happened a handful of times.

I wish I could remember how the seven-year-old mind thinks, because I'm sure this is a great game, but my adult brain just can't seem to figure it out.