Sorry to hear about this issue! If you lost all your coolant in the span of 30-45 seconds, and you were on the highway with coolant system pressurized, the front or rear coolant pipes are prime suspects. The fact that they were just replaced is odd. If it was a loose clamp, I'd suspect you would have seen or smelled coolant in the months you drove it. Is it possible that the catastrophic leak came from where the front coolant pipe inserts into the top of the motor? I think there are two pipes with o-rings (at least my '10 Range Rover 5.0 did) and perhaps those weren't fully seated? I know when I replaced mine, it was difficult removing those pipes. Or mabye they accidentally left an old o-ring in there when inserting the new one? It's the only location I can think of where the mechanic may not have looked to find the smoking gun since they'd have to do some disassembly. Best of luck!