I had the same "Engine system service required" pop up a couple of times that later went away. I also was having intermittent cruise control issues, in which i would engage, but disengage after a few miles or it wouldn't engage at all.
One day, the message came back and didn't go away on the next couple of starts, so I dug into the problem a bit more. I did the trip-meter-reset thing to see if it went away, but it didn't, but it did show a 44 in the message are. This eventually brought my searching to some forum post where they mentioned a link between failing or intermittent cruise control and this message. The solutions"?
1) Take the brake pedal position senor off, take it apart and clean it. Something I knew I'd mess up
2) Get another sensor from a junkyard (I got mine out of an S80) it cost 18 usd
3) Get one online, it goes for around 100 usd
I choose #2, if you google for method 1, you'll see why it fails and it made sense when comparing the sound of both sensors, my old one was really scratchy, while the "new" one was a bit smoother.
Anyway, I replaced the sensor (getting the clip that hold it in place wasn't too intuitive for a noob like me). Now, no more messages, I've driven it for about 800 miles after the repair, and the great thing is, I revived the cruise control!
This sensor is also called diaphragm position sensor, it sticks out of the booster, on the passenger side of the housing.
This was on my 2001 V70 T5 (P2), btw
2001 V70 T5 - 150k miles and going strong