I've always found it strange that she called him at the pay phone, then called him on his cell that he supposedly didn't have, where TM might either answer or know he answered.
I've never seen this as a neat and tidy scenario where SM either (suddenly) called Heather at that hour and announced that he was leaving TM, or, where he (suddenly) called her to "lure" her to the landing. There are too many other moving parts to the story between the pay phone call and her car ending up at PTL.
Since I don't believe a murder took place there and I'm not sure there was a "lure", someone sure was was in a hurry by landing time, including Heather who made three back to back phone calls in a span of 3 minutes or less after pulling into the landing.
I always come back to the question of at what point did she lose control of her phone and her ability to operate her own vehicle. I find it hard to believe she arrived at PTL and hopped out of her car, making it easier to grab her or whatever supposedly happened. I can believe a scenario where at gunpoint she was told to get out, although time was short at that point. But given how she was parked, I'm not sure she couldn't have escaped in her car. Even if we account for her being so frightened that she did what she was told, did she have the presence of mind to gather her purse and phone, get out, lock the car, then hop into the tool box or the cab, leaving no trace of having been in the truck? Or did one of the M's grab her possessions from the car, have the presence of mind to lock it (why?) and not leave a trace of evidence that there was an encounter at/in the car? That was one fast and smooth transition by both the M's and Heather.
I just think there's a chunk of the story that's missing and the perceived chain of events is not quite what happened. And the answer to 'Where is Heather?' is wrapped up in that somewhere. JMO