I was recently swimming in my pool and happened to notice a rubber plug in my main drain housing. I dove down to inspect and thought that was weird and removed the plug. I am a first time pool owner and the pool is about 30 years old and well maintained. I just moved in this past November. I opened the pool myself this year and am wondering if it’s common to plug the main drain for winterization. I live in Missouri. Or do I need to worry that there is a leak in this line and it was plugged by the previous owners. Or could the drain housing have two ports and I removed a plug that shouldn’t have been. Any help is appreciated.