Ben loves him some fast food! The actor seems to be going through a "phase" right now -- he's smoking again, going to drive-thrus every day, possibly cheating on his girlfriend with a Playboy model, and seemingly pissing off estranged wife Jennifer Garner, obvi!

So here's the dirt -- Ben's back in rehab and Jen's behest, as we know ... he's possibly cheated on possibly-pregnant girlfriend Lindsay Shookus with a Playboy model in front of ALL the paparazzi by having his "secret" rendezvous at Nobu Malibu, the most celeb-frequented, pap-infested restaurant in all of LA. So what gives? Well here's the EXCLUSIVE scoop ...

Our photographers spotted Lindsay Shookus, who's been in LA but hasn't been at Ben's house, as of late, driving Affleck's Bentley on Saturday. Okay, so why would she have Ben's car if they've broken up? Well, she had the car for only so long. Lindsay drove the super-expensive ride to a parking garage in Santa Monica and left it there! She never came back for it! She took off in a friend's car and left Ben's car in some random parking garage!!!

We don't have Sherlock Holmes on staff at X17, but we're pretty sure a move like that means there's been a breakup ... a nasty one!