A lot of the ones I've noticed looked pretty clearly on a higher stage of an ascent, but I've also rarely noticed zig-zags. They could be fighter jets, which fly around a lot more than most might think and aren't super predictable in what they do.

Still though, I'm checking out of this discussion now. I'm not sure there's an on ramp to convince me, and it's because of what I said before. I just do not have the mindset to sprint to maximum complicated conspiracy when the easy solution is feasible. If the government/world were to alter weather or poison the populace or something, they could find much simpler ways than a complex conspiracy to secret manipulate passenger airplanes for decades to distribute whatever. Sometimes the answer is the obvious one, not the full tinfoil hat.