Ben and J Lo caught a showing of the Timothée Chalamet and Zendaya-led sci-fi flick Monday night in Los Angeles ... but the fact the famous couple squeezed in between a bunch of regular folks isn't the most interesting thing here.

A group sitting behind Ben and Jen took a cell phone video of them leaving their theater seats -- and you see them picking up their trash ... namely, popcorn buckets and soda cups.

Check it out ... Bennifer (but mostly Ben) gives their seats a good once over to make sure everything is spick and span before heading out. It's certainly good etiquette, we'll say that.

Ben and Jen kept a pretty low profile here too ... people in the theater say they didn't even know the couple was sitting there until the credits rolled and the lights came back on.

Also ... it seems J Lo barely touched her huge popcorn bucket too ... and they opted for a regular container, not one of those viral 'Dune 2' ones that have been making the rounds.

In any event, it looks like Ben and Jen left no trace as they exited ... which can't be said for everyone who goes to movie theaters in America these days. A lot of slobs out there, as we all know very well.