could only validate if the gaskets are all good with water test

not long ago, my master "small second" (L2.676) was having watermark in the middle of the glass although I don't clean it with water or even wear it under the rain; the only possible that the water slip-in was during normal handwash.
It's not too bad because lucky it's still under warranty, they SC fixed it.... but I still wondering how that could happen to a new Longines master series watch.
