That description of the size and the way it just held position without any sound - that's the part that gets me every time with these reports. The treeline context really helps with scale estimation, so that's useful. How long would you say it was stationary before it moved or disappeared?
I had something similar out here in the PNW about three years back, big dark shape, totally silent, low enough that I expected to hear rotors or anything mechanical but there was just nothing. The absence of sound is almost more unsettling than the object itself when you think about it.
What direction were you facing roughly, and did you get any sense of the object's shape beyond just the mass of it? Triangular, rounded, or more irregular? Those details matter more than people realise when you're trying to narrow down what category of phenomenon you're dealing with.