It's an air bubble. After I topped off my coolant tank, the air bubble causes it to use the coolant overflow above the passenger rear wheel. Unfortunately, all of my coolant drained out, if you level is still okay I wouldn't worry about it.
It is not a problem, or a big deal unless your coolant light flashes or the level is below the min. mark. There is no need to go to a shop if this is the case, because they will charge you and fix nothing, like they did to me.
Relax!
