Eh...
Yes you can detect empty vehicles... Easily...
a)
Empty vehicles are set to side Civilian. OFP engine does this. So if using triggers to detect empty vehicles the trigger should be set to be activated by civilian...
b)
When detecting something with the binolars, the command knowsAbout comes in extremely handy...
You name the NATO trucks, like truck_1,truck_2, etc. etc....
Then, make a trigger:
size 0/0
activation: none
condition: player knowsAbout truck_1 > 2 || player knowsAbout truck_2 > 2 || player knowsAbout truck_x > 2 etc... (repeat this to all the trucks...)
On activation: what ever you need to make the mission end...
Now, this trigger 'fires' when the player's knowsAbout value goes over 2... You may need to test around with that value, maybe less will be better...