I’ve had my NC 7 years, I found one drain hole around 3 years ago but never the other until this year.
All ok drains running freely (I check regularly) until the said one I couldn’t find began to trickle out more slowly this year. Determine to find it I must have spent 30 mins faffing around until I decided to as I said above, get a torch and have a closer look, bingo I hit the spot.