Fish'n Jim wrote:
I'd take it out and examine to see what it might be and what's behind before drawing conclusions.
Without documents, no telling who or why that's there. Usually when there's only one, it's not factory, unless an installer had to solve a pucker, wrinkle or similar issue. Can't be certain some repair/replace was done.
Some corrosion in the area maybe related but visual circumstantial speculation.

ps: Lots of fading in that cloth. It'll also appear different if you launder (Woolite) a swatch of the dark under trim piece. Filters/collects dust, etc.
People do a lot of weird things and maybe an owner stuffed something. Part of the unknown vehicle history. No internet answers for that.