Drains and returns

I bought a house 12 years ago that has an 18,600 gallon free-formed inground pool. The floor drain had been sealed off. Why would someone do that? Did it have a leak possibly? Also, it appears that there are 3 returns, but one of those has always been plugged as well. I say that, but I'm not even sure if they are all return lines. The one that is plugged has a white collar around it while the other two don't so maybe it's not the same thing. What else could the third opening be? Any thoughts?