Where was this speed trap?

Last week, I was on my way to rehearsal as usual on a weds evening, around 6:30pm south on 270. Surprisingly few cars and it was against traffic, so I merge up onto the express lanes, putting along at oh, 65mph (X3 doesn't have torque to zip up quickly) in the right lane. I pass a known speed trap and do spot an unmarked crown vic on the left shoulder, also facing southbound. I didn't really panic for some reason, maybe cuz I always thought they only shot radar guns forward before the vehicle passes them, not AFTER the vehicle passes.

A couple seconds after I pass the cop, the lights come on and the cruiser starts coming after me. My heart freaking stopped and like, fell thru the floor cuz I thought he was coming after me and I'm just 2 months shy of getting a full license (I guess 65mph isnt 55.....). I slow down a bit but keep going. Eventually he nabs the only other car around, a Chevy Aveo which was actually slightly behind me in the middle lane. Whether that cop was coming for me, I'll never know. But that was a shocker and kind of ruined the evening.

On a side note, anyone know how they use those radar guns? I have a front nose mask and lack a front plate, so they can't really get a reading off the front. Do they aim at the side mirrors? Can they aim at your back after you've passed?