My volvo xc70 low coolant warning on again.

It was on six weeks ago. I looked at the it and it was just below the Min level. I called the dealer that they said just adding prestone coolant so I did. But this moring it was on agin when I started the car. It was off after 10 seconds as the car warmed up. Could be a leak? Should I take to the dealer since it's still under warranty? Thanks!