What is this on my phal!?!?

[I.heart.orchids.]

Hey everyone, I'm new to the forum but I figured I would ask rather than waiting to see what happens and stress out.

I just discovered this morning that one of my phals has developed a brownish irregular spot on the underside of one of the older leaves, the top of the leaf is fine, there is no yellowing of the leaf. The spot is dry, doesn't smell. I water the orchid only when it's practically dry with a very diluted organic fertiliser, It gets only morning sun, filtered by a curtain ( as it has scars on the leaves from when i bought it, most definitely sun damage). I would say it's sunburn but I can't be sure as it's in such an odd spot. Perhaps cell collapse? I went away for 4 days and left the window open to bring some fresh air into the room. I can't say at this point if it's getting worse as I only saw it a few hours ago, but I'm freaking out! I've posted a photo, the spot looks black but it's actually a very faded brown colour and the surrounding area is dark green, almost as if the orchid leaf has been physically damaged. I know what rotting on an orchid looks like and this just doesn't look like it.

Please can someone help me figure out what is happening to my orchid.

Thanks!!!