Check coolant level??

[Stevo///M3]

I have an e36 M3 evo and when I turn the car off the trip computer warns me to check coolant level - see owner's manual. I'm no mechanic, but this only happens intermittently - it doesn't show EVERY time, only sometimes. I recently took it to a garage and asked them to check, which they did, but the warning still comes up every now and again. Has anyone else experienced this and does anyone have a suggested solution? The indicated level of coolant is fine (shows between max and min under the bonnet in the little overflow tank near the driver's position)

Cheers
Stevo

[Stevo///M3]

Thanks each of you for your helpful hints - sounds like i could do this myself one Saturday afternoon. Faith in internet chatrooms restored, cheers!
Stevo