Addendum.

I've noticed lately that (younger) customers of all types (Asian- and Caucasian-American, South Asian from Asia) do this thing where they have separate bills, and then someone decides to pay for someone else and takes the check of that person and slips it in behind their bill and their credit card on their black plastic tray, all without telling us, as if we're carefully looking for nestled receipts when we don't expect them, especially after we've just received instructions to subdivide the bill in a different fashion.

I wonder where this goes back to -- perhaps a TV show somewhere, where they've seen someone paying for someone else's bill like that?

Like, I've never seen it from (older) customers, and it spans both (American) and (non-American) (young) customers, so it must be coming from somewhere, and the only thing that I could think of that could link those groups would be mass media.