Thanks for the response, Kenny.
Don't know if you still remember that the refrigerator is a french door model from Samsung. I can still remember how it was fixed at the first visit. The drain hole is inside the fridge but completely behind a big panel on the very top. There is no chance it can be plugged by food debris (plus I keep a decent clean condition in the fridge). The turkey baster with hot water was used to flush and melt the frost last time. The exact symptom happend for this time - some water ponding in the right bottom storage drawer.
I appreciate the professional and courtesy service for your first visit.