Plus, didn't they supposedly take their guitars so they could play at a fire? You certainly could carry one down to the other field but I'd think you're on the right track with them just sitting at the table.
In the news photo I posted, do you think that's their car? If so, they were backed in. If they'd been chased I don't think they'd have backed in. I can't even imagine them pulling in the lot at all under those circumstances. If the killer was already there, would they have felt a bit nervous about being in a remote location with little daylight remaining? Sunset was at 7:59. Murders were estimated between 8:25 and 10:00. I assume somebody must have been along there at 8:25 for them to come up with such an exact time.
So they would've been losing light at 8:25. And it's pretty remote out there. I think it would feel creepy to me. If a vehicle was already there, would they have stayed? Or perhaps parked to talk it over? It's hard to imagine they'd have pulled in close to another vehicle and we're told the shootings were close range. Plus, if David was shot first, the killer had to get to Heidi pretty quickly. That would indicate they were close. If they couple was preoccupied, perhaps somebody could have quietly approached the car.
But were they already at the table when somebody arrived? That could have triggered some nervousness and persuaded them to leave. If the person parked near them, the couple might walked right past them.