I remember playing against you rather than with you at one point — you can be a serious pain!
I wouldn't really think you suspicious for finding enemy in treeline. Players (especially snipers when their team is in trouble) like to congregate there. It'd be my first go-to spot and, even as your gunner, if you hit someone there unspotted, I'd probably just shrug it off as luck as there's almost always someone there from the Russian team when they don't have base B.
When I was gunning for ULTRA, we were flying over B facing straight ahead. The first infantry he went for were on right hills maybe half way from treeline to B. When he turned around for the others, they were left of B wind turbine between the trees and start of UAV hill. At no point even after he started shooting they were spotted. My first instinct before shooting is to spot, but even as he was shooting I couldn't see anyone. They were not shooting so there were no vapor trails to give their location away. Those two spots are just way too random for someone to shoot and hit someone by pure dumb luck. I wouldn't have been so certain he's using something or even suspected him otherwise.
I saw that ULTRA is a username on the forum when he signed up to appeal a ban (for server disruption. it's what Admins use for users suspected of using hacks). I think him seeing this thread will ruin any chance of us actually catching him in the act. He may be careless but I doubt he's stupid, so he'll probably be more careful playing on in squads or partnering with someone in heli.
It just hit me that you are minichaingun. I've played with and against you. Wasn't there another player with a similar name you used to play with?