Osmotic blisters ????
If so it is not good but is seldom the end of the world and likley no know cases of catistrophic failure.
Dig out the void and look at the extent to check that it is a 'pocket' and not a wide/far ranging delamination.
Get copy of west system epoxy's pamphlet on blisters and repairs. It is cheap, comprehensive and published by well know and well respected folks. LOTS has been written on blistering.
If it not blistering but a delamination/structual issue (probably unlikely) then start by surveying with a plastic/wooden mallet and sounding out the void by tapping.
Luck to you.....