Odd stain on Oncidium leaf

[Karen00]

Hello folks,

I'm looking for a little help in trying to identify what looks like a stain on one of the older leaves on my Oncidium. This is a mini Twinkle. I purchased it in February. Currently it's growing outside.

Characteristics:
- The tip of this leaf was broken and dried when I got it. I'm not sure if a virus, bacteria or pest had already infected the leaf. I don't recall whether this stain/spot had already started. If it was there it was tiny.
- I think I first noticed this a couple months ago and thought maybe it was sunburn when I first put my orchids outside.
- This is on one of the older leaves. All new growth is fine.
- Colour: I can't tell if this is brown in colour. It looks kind of red or orange but that could be the colour mixing with the green of the leaf.
- It looks exactly the same on both sides of the leaf. Unfortunately I don't know if it started on one side or the other or both.
- It's not appearing on any any of the other leaves (so far) and the pseudobulb is fine (so far).
- The leaf feels perfectly fine and it looks healthy otherwise. It's not like it's dried or mushy or pitted.
- When I was feeling the leaf I thought I felt something hard on the underside but I couldn't see anything. I wiped the leaf down with isopropyl alcohol and whatever it was wiped off. Not sure if it was a bug or anything like that.
- It's spreading very slowly but is spreading.

I have never seen anything like this before. I couldn't find anything in any of the searches I did.

As mentioned I wiped the leaf down with isopropyl alcohol. I'm not sure if I should continue this or just cut the leaf off or just leave it if this is the normal demise of the leaf.

If more information is needed or more pics let me know.

Thanks in advance for any help!