The evaporator coil is in the freezer. When it goes into defrost, that water should go down a drain in the freezer into the drain pan. Open the freezer door and see if there is any ice in the very bottom of the freezer compartment. If so, then the drain may be clogged and will have to be cleared. It is necessary to remove all of the racks and baskets to get to the sheet metal that covers the coil. Remove that panel to get to the evap coil and look below it to find the drain. It may be necessary to use a hair dryer to CAREFULLY melt any ice. Be careful not to melt the plastic liner. When you think you have the drain cleared, check it with a large glass of hot water.
If there is no ice in the bottom of the freezer, take the grill off the bottom of the fridge to see if there are any leaks under the fridge.
Could also be a pin hole leak in the water supply line to the fridge.