I am surpized you found no water in it. You might be right that the moisture made it come loose. Now the questions are how much water leaked into the ceiling and if it didn't leak down stairs where is that water and what is it doing there. Wet drywall is a great food for mold with all the problems that could cause. This bubble will not go back in place and will have to be repaired, with that in mind I would remove the bubble and inspect the material of the ceiling. If you dig thru the paper a little and it is dry, white and chalky you will be fine. If it is wet and mushy it should be removed and the area should dry out before patching is done. If you have plaster in an older house, it does not feed mold as bad and it may dry out on it's own.