I just re-watched the episode during my work out and my initial thoughts were correct.. They come up the On ramp, while moving the cars Michonne see's the trap.. while they are looking over the trap Carl ( or Coral depending on your punctuation

) mentions that while hiding in the truck he overheard people talking about a "herd Trap".
After the Slice & Dice, they continue on the way they had been heading before they saw the trap.
Also, Re-watched closely the beginning when they went back and showed the clip at the end of the Mid-season break..
Boots watched Gab, and then walks towards Alexandria.. When this episode starts, Gab is up on the lookout now, keeps looking down, looks at his bible and shakes his head "yes" at which time he starts loading the car.
I am assuming that between the end of Mid-season and the start of this episode Gab talks with Boots and comes to the conclusion that they need the help and need to help others.
And yea, sure enough, as he is driving away you do see a head pop up in the Passengers seat ( back seat was full of supplies).