To me, that guy doesn't look very suspicious, just another tourist. Could Lars have made a sound, maybe talked out loud to himself, that made blue shirt guy look over? That's kinda what it looks like to me.
I noticed that when Lars gets outside, in the footage that's seen towards the buildings, he initially goes straight/to the left, then seems to make a sharp turn to the right (from our viewpoint). That change of direction seems to be right after the point where he puts his hand on/in his left trouser pocket.
According to Google maps, the car rentals are in the part of the airport where Lars comes out. Did he have a rental car? Just thinking that maybe he parked the rental in that parking lot and either a) thought he still had the key to take off in it, but he didn't, so he changed direction; or b) he still had the key but saw people (or thought he saw people) by/near the car that scared him so he changed direction because of that.
Or maybe a bus ticket? There seem to be buses there as well in the CCTV footage, although they look more like touring cars than public transportation buses (although I have no idea what those would look like in Bulgaria). Either way it looks like he suddenly and quickly changed his mind out there.
The direction he headed does not seem like a thick, big forest to me, when I look it up on Google maps. Smaller trees and larger shrubbery in places, no doubt still good to hide in, but it changes into an open field pretty quickly, with the highway right by it. He could have easily hitched a ride there. IMO
