Most modern window air conditioners create condensation which drips down to the pan on the bottom of the unit. There are no drain holes or plugs used to remove the water as was the case with older style window AC units. When the water builds up it is distributed by the fan sling and thrown up against the condenser. This helps lower the air conditioner's temperature making it more efficient. If the amount of water is high the fan may create a sloshing sound as it makes contact with the water. This is the normal operation. If the water level gets too high it will drip out the back side of the AC unit. Make sure the AC unit is install properly, with a slight angle so that the condensation drains towards the rear of the unit.
Some 5K and 6K BTU models will have an optional drain hole in the base of the unit. If the drain plug is in place excess water will drain from the back of the AC. If the drain plug is removed the excess water will drip from the drain hole. Either way is acceptable. Even if the drain plug is removed a separate reservoir will still capture some of the water so that it can be used to by the fan sling to cool the unit.
At the end of the season the excess water should be removed from the base before proper storage. Appliance service providers can assist in this process during regular season maintenance.