Ok, I'll check this out further. I'm sure that is it, though.

It sounds right....the application pool has recycled and cleared out the MAC address list.  If you connect to the ES, and then refresh the page, you will see that your MAC address will be in there.  It is not predictable as to when the application pool may recycle .... and .... if someone was connected to the ES when the application pool recycled, then it would not reset that list as it would be in use.

I was trying to use the ES status page to see if any users had connected to ES...which I know many have, but it shows that 0 users have ever connected. This doesn't seem right or very useful