The red secret hidden within Mystic Ruins' past

(18/08/2023) Edit: It's been solved!
Big thank you to AlonWoof, for checking and confirming what that is.
It's the smoke effect from the main Mystic Ruins past hub area.
Check out her video showing it off here:

Original description:
I've reached a new low when it comes to obscure content.
This has been at the back of my mind since I played the game in the mid 2000s.
I've always noticed this, and I'd love to have closure.

[AlonWoofPro]

Pretty sure those are smoke particles, the task probably didn't get fully deleted by the time the other act loads.
I could look into it directly a bit later today if you'd like. ^_^

[Royameadow]

Could be the Flame Texture from Last Chapter (the Super Sonic campaign), perhaps there is a flag that determines whether of not the Emerald Shrine is supposed to be on fire and begins this process by having the flames turned on before setting them to be disabled; we would need to probably have a more optimal Debugger Mode to analyze this better, but this is definitely the most believable scenario, though now this makes us wonder how the place would look like if it were burning during the day (hint, hint, modders).

EDIT: It may also be the Fog/Haze that is meant to be present because of the flames in Last Chapter: Notice how all of the mountains have a Red Glow to them when in this condition; the same theory as above still applies, the game may be coded to assume this condition first before disabling it and the Rising Red that we see here is simply the Fog being disabled, would make sense due to how it affects literally every object that is out of the tunnel.

[mighty_king]

This needs to be discovered and unearthed immediately

[thatitalianlameguy]

That just looks like the same red light coming from the previous area spawned for a second in the other area

[yellowinahat]

It's like drawing a non-perfect circle.

[gabrielpitonidedeus]

I think that Sonic Adventure is better than Big Rigs

[Dreamcasttime]

I think i can tell. Remember the Sonic Flashback from the last story? I think game loads the color pallet of this area for a second, and then uses the intended color scheme

[sonicthehedgehog]

What if that was sonic.exe but as a spirit

[Ezpeezy]

I found a collision error on the lower part of speed highway when you enter the tube just as I found this video

[tiff]

This is quite a clickbaity title. It isn't a "secret" nor is it reasonable to presume so.