Have the system checked annually.
For now however, I suspect that your condensate drip tube is plugged and the drip pan is holding condensate. This condensate tends to be laden w/bacteria and has a rather moldy smell to it (a definite health concern if not addressed). This pooled up condensate is being (recycled) picked up by the air moving across the evaporator coils and back into the living quaters by the blower.
The fact that you dont' see anything leaking is actually a problem. It should be leaking, leaking condensate. Wheather this be from a weep hole in the unit or from a drip tube, you should be discharging the humidity from the air that's being cooled.