You dont need to have them act under fire automatically or always.
First find out what it is they want to be doing in response, give them the fictional move.
And if they say something crazy like, they're going to charge the bullets with a crowbar and screaming ARR-ARGLE-BARGLE!!, say well, that's going to hurt. You'll be acting under fire if you try it, and you'll probably suffer 2-harm unless you're using cover. Are you sure that's what you want to do? If they say something smarter like, I'm going to drop the goods and make around the side trying to get into position to hit them back, maybe scare them off, cool, they act under fire to get into position, if not, NOW they're the ones being seperate / chased off. Or hard bargain might be taking some harm with them too, or giving up on that whole idea.
If all they wanted to do was try to run off with an arm full of scrap, maybe they answer is just SURE! You got what you grabbed and take off running, but most of your scavengers are running off hands free and maybe "bob" starts to scream when a bullet rips through his knee and junk goes flying every which way. What do you do?
And if it's Escape, ask yourself, okay, is that car really focused on this PC? Or the land? Or is chasing them off the objective. They've got wounded to interrogate unless the PC saves the guy, so... the snowball is still in play. Maybe that's enough for now and you let your guy escape, and you've got a bunch of scavengers who might be hurt, mad, and are most assuredly afraid. That's a good immediate place to pick up, round a few corners or hell back at the trucks / back in the hold / whatever just so long as the car threat as past, maybe whereever feels like a safe place to recoup.