The distant AT Guns wish to skeedaddle....

[Blackcloud6]

8.22 says: Because spotted units lose their spotted status when they leave their original hex and enter another limiting terrain hex, they cannot be fired upon with opportunity fire during this movement unless the firing unit(s) can spot the target in both hexes. 

So I would say the guns remain spotted. I don't think the spotted status would transfer to the trucks as there is no provision for such a transfer in the rules, but it does not matter as the guns are still spotted until they move away and the bad guys can fire on the hex affecting all.