The creature was back yesterday. We couldn't leave the cave for very long, as on our way to the door we spotted it walking slowly through the woods. It didn't see us, or at least didn't pursue us, as we made it back to the cave safely.
We ventured back out today in search of more firewood, being very careful, and found the door again, which is odd because we weren't exactly searching for it, and it seemed to be in a different spot than last time. At any rate, we inspected it while we could.
It is still locked, but there are some new developments. I found an image-creating app on this phone and used it to sketch a few of the things on the door. First, the imgur link that was hidden in previous posts does display the correct symbols on the door; but it's only the top row. The top three symbols are in large, black print above the door.
Second, the triangles.
This is what they look like. They haven't moved and appear to be in the same condition as they were in our previous encounter with the door. Still not sure what to make of these.
Third, the paint on the door - it's a deep blue - has been chipped off near the bottom, forming this symbol.
Once again, not sure what to make of it. I've never seen a symbol quite like it, though it appears in some ways to resemble an hourglass. I don't think it's Japanese script because it's been scraped into the door while the script is written in black.
One final observation - the doorknob is loose. Last time it was pretty rigid, and while it is still locked and secured to the door right now, it is a fair bit looser than it was before, as a knob gets with heavy use.
Serah and I had to run back to the cave as night was starting to fall. We haven't decided if we should go back or not, and we're having some slight disagreements on how to proceed. Serah wants to bash the door open, while I fear that may disrupt whatever purpose the door is intended for. I have some feeling that this is our way home, but I can't be sure how.
Will update when I have more information to share. Stay safe, and let me know if you see any instances of the creature or something like it near you. If he's not here, I don't want to think what he must be up to.