Yesterday, a warning light lit up on my 1998 S70 T5 dash that I have not been able to figure out. I did not find it in the owner's manual. It is in the second light from last position on the bottom right side of the indicator panel on the right. It is an orange color triangle in a green square with what looks like an inverted "Y" inside. It lights on start-up and stays on when driving. I cannot figure out what problem it relates to, as the car drives just fine.

Can anyone help?

This light often light when the gas tank cap is not properly screwed after a gas fill. It will light after half a tank is gone, and will likely stay on until you have it reset by the dealer. Since I own my car, it happened to me about 5 times.... Now I try to go only in garage where we gas ourself!!!

If you're in the US, then you've got a Canadian-origin model.
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David \\ (98 S70 T5SE Black, misc mods (mostly lighting), red calipers) (92 940GLE)

That's the Lambda sond or "check engine" light on a Canadian car (possibly other markets too if the car's not from Canada).
In a US car, the same light says "check engine" and with your S70, a scan tool is needed to read the code(s).