I'm trying to help out my son with his 2000 S80 T6. I don't recall exactly what the text is but it is the equivalent of a general message like "service the engine soon". It does NOT refer to anything specific like ABS or SRS systems. After some driving it goes out. Then returns. Nothing is wrong with the car, the fluids are up to proper level. I am left wondering where to find the reset button, similar to the one that I am friendly with in the 200 series.

put the ignition key in position I, push and hold in the odometer reset button, move the key to position II (don't start it), wait until you see the service light blinking and release the odometer reset button. --

His issue is not the service message...it's a diagnostic code issue.
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1998 S70 T5 Emarald Green Metallic (125K), 2004 V70 2.5T Ruby Red (45K), Previous Owner of Black '94 850 Turbo Wagon (Sold at 140K and miss her). My cars have been running so well lately they've got me worried.

No reset button...you'll have to get your codes read to see what's causing this. This isn't the typical service message (to get your oil changed). Most autoparts stores (autozone) will read the codes for you. Write them down and post them back here and we'll help you.
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1998 S70 T5 Emarald Green Metallic (125K), 2004 V70 2.5T Ruby Red (45K), Previous Owner of Black '94 850 Turbo Wagon (Sold at 140K and miss her). My cars have been running so well lately they've got me worried.

RE: Have codes read: THX for the tip about where to get, I assume, free readout of the OBD. I had no idea the auto parts stores did that. It may be a week or so until I can get my son to swap cars.