There is always CO2 in the solution because of fermentation. If your fermentation was done on the cool side more CO2 will be left in the solution. As it warms up more CO2 is released by the solution and, therefore, will be flatter and will require more priming sugar to carbonate it.
So, if you fermented at, let's say 60F, then your beer may have a good amount or CO2 in it. I made an amber ale that fermented at 60 and I remember seeing quite a bit of bubbles and foaming when I would take a gravity reading.