The alternative, "You and that mouse in our pocket," and " You and that invisible mouse in your pocket," is in reference to a speaker talking in the plural tense referring to themselves for a variety of reasons mostly to make them seem more important or that it involves others involuntarily. It is an attempt usually to pull others into a situation or to act as if other agree with them against their wishes or having spoken with said person/ people before hand.
This is not to be confused with a Pocket Mouse
It also can be used to make someone seem as if there is more than that person. See fronting
Mouthy drunk in a parking lot shooting his mouth off: "We are going to kick your ASS!"
Man who is being mouthed off too: "We? We who? You got a mouse in your pocket?"
Example 2: Picture four mechanics on a break with work orders on the lunch table in front of them. Then in walks a service writer who immediately says "We need to get on that Transmission in bay six, PRONTO!" All of said mechanics have work already required of them making one of them ask, "We got to get on that truck in bay 6? Who is we? You and that invisible mouse in your pocket? We already have work orders assigned to us."
Eric: I hate that ginger
bastard. Let's go kick his ass.
Eric: I DON'T CARE WHY! GINGERS HAVE NO SOUL! WE REALLY SHOULD KICK HIS ASS!
Kenny: M mmm mmm mmm Mmm mm m Mmmmmmmm? Mmmm mmmmm mm mmm m mmmm. (TRANSLATED, You have a mouse in your pocket fat ass? I am not going to kick anyone's ass for you.)
Everyone laughs and Eric shouts:
I'm not fat, I'm Big Boned
Jerry Brown: "We need a High Speed Railroad in California. The People demand it."
Heckler: "You got an invisible mouse in your pocket? I don't need no high speed train to nowhere."
Supervisor :"We really need to clean up this trash."
Workers: "We? Since when did you and that invisible mouse in your pocket do any of the work here?"