Right so I need more detail before I can even start theorising here. What time exactly? Because there's a massive difference between something spotted at dusk when your eyes are still adjusting versus 2am when the sky is properly dark. Also what was the light behaviour like - was it steady or pulsing, and did it move in a straight line at any point or just hover? The treeline thing is interesting to me because I've had two reports in my files from people in the northeast of Scotland describing something low and slow just above tree height, not gaining altitude, just sitting there.
Was there any sound at all? That's usually the thing people forget to mention and it ends up being the most useful bit of information. Drones make noise. Most conventional aircraft make noise. Some of the more credible UAP reports specifically note the total absence of sound and that always gets my attention.
What county are you in? Might be worth cross-referencing with other sightings in the area.