Jimmy moving his seat and seeming to blatantly stare at the guy, and then Gus approaching him when he was digging through the garbage had me thinking that Gus was on to him. We know how observant Gus is. He knew Jesse was high before his initial meeting with Walt/Jesse before he ever went over to Walt. He damn sure had to notice Jimmy changing seats and staring at his guy.

Gus taking out the trash(I think that's what he did when he went out there) when Jimmy was sitting with Mike in the car confirmed it, IMO.