Air Conditioner - water standing in condenser housing
A drip pan backing up is very common.
Look for the drain, there is probably some growth of moss / mildew and dust growing in the drain. Don’t use vinegar to clean it vinegar will Dissolve the galvanized coating over multiple uses and is hard on the coils.
The drain is usually on the outside edge and I always tilt the pan so that is the low point.