First of all, I want to strongly disagree with the use of Act Under Fire as a saving throw, since that's not what it is. The MC doesn't tell the players what their characters do, just provide them opportunities to react. So you should never say, "You feel a sense of unease; roll AUF," and then tell them what they do based on the roll. Instead, say, "You feel a sense of unease [or whatever]; what do you do?" and then whatever they say their character does might be Acting Under Fire (if they say they drop to the ground, for example) but it might also be something else. (E.g. "I ignore it" is probably a golden opportunity for you to deal harm, no roll required.)
Now, that said, I understand your impulse to want to preserve the surprise, but I'd say forget about that. It isn't your job to surprise the characters or players. Rather, your job is to be a fan of the characters, make their lives not boring, and play to find out what happens. You have to at least implicitly tell them something bad is going to happen, because simply framing the scene does that. Unless the very first thing that happens in the scene is them getting blown up, the fact that you didn't skip ahead to the end of the journey implies that something is going to happen, and since this is AW, it probably isn't good. They're driving through bandit country, though, so it makes sense that they'd be on their guard anyway.
So go ahead and start off with that nice soft "announce future badness" move. What's the tell-tale sign of these explosives? Maybe the road, usually hard-packed dirt baked dry by the sun, has a dark patch where it's been dug up ahead. Tell the Driver that and ask, "What do you do?" Then play the scene out from there. If they keep driving, boom, inflict harm. If they stop and get out, maybe the bandits attack or maybe they set off the mines themselves, depending on their actions and the results of their rolls.
ETA: The "future badness" can be even earlier, too. When they set off, let them know this trip is going to be dangerous (they've heard rumors about the bandits, have had confrontations with them before, or just see the burnt-out cars along the road) and ask if they plan to do anything about it. Then use whatever they say going forward. "I'll keep a close eye for anything out of the ordinary" might qualify as Read A Sitch, and a successful roll will let them see the trap as they approach (any of "what's my enemy's true position," "where's my best way past," or "what should I be on the lookout for" will work here) and a failed roll will lead them straight into it. Maybe they'll reinforce their cars with armor before they leave in the hopes that they can plow right through any ambush, in which case you'll deal harm as established and see if they were right. Maybe they'll send one car up ahead of the caravan to trigger any ambushes, which the rest of the caravan can then go around, in which case you deal harm as established to the scout vehicle and ask what they do with the rest of the caravan. Or heck, maybe they'll just decide to go the long way 'round, through the burn flats instead of the marginally more habitable land around the main road.