Used to be that the cap on the collant developed a crack which caused the coolant level to slowly dissipate and eventually cause the warning light to go on. Volvo ended up changing the cap. I would check it to see if there is a crack and whether the level is going down because of it. Other than that, you may have a leak somewhere else. The light will go away because pressure builds up, but I would still check to see if any other leaks have occurred.

Delgado
San Diego