Possibly the cells had vented previously and the seal is now open. I would throw them away, then wonder why they'd gotten hot enough to vent previously. Some Delta-V chargers only have a Delta-V detection but not temp monitoring, so if a cell were fully charged and an event like AC power flicker occured at long enough duration to reset the charger, it might then start charging at the fast-charge rate again, but there will be no Delta-V to detect since the cells were past that stage already... so if they get too hot, they vent which was better than an explosion.
At the very least if you continue to use that charger, you might not want to fold down the cover to increase passive cooling. There are other things you could do like pointing a fan at the cells but it seems a hassle instead of just seeking a different charger, IF it seems the charger doesn't have the temp cutoff feature.
No with undamaged cells and proper charging (not overcharging) there is no gas escaping to be building up in the compartment.