That feeling of not being able to stop thinking about it is honestly what gets me every time. Your brain keeps replaying it trying to find the logical explanation and when it cant, it just loops.
What was the movement like? That's always the detail I find most telling. Slow drift is very different to a sudden directional shift. Anything making sound? I do a lot of EVP work out on the moor and you get so used to the usual night sounds that when something just feels wrong and quiet at the same time, it sticks with you in a way thats hard to describe.
Did you have your phone on you? Even a shaky clip helps when you're trying to reconstruct what you actually saw versus what your memory fills in afterwards. Memory is unreliable, especially under that kind of stress.
Would love to hear more detail when you're ready to write it all out. The tree line stuff round here always gets me - there's something about that threshold between open sky and woodland that seems to attract this kind of thing.