It certainly was rebooted, but this is something different than cache slowly creeping up. Memory usage is still low since I originally posted this message.
Hopefully this will continue. The only logical explanation I can think of is that the VPS Spirits suddenly felt I had been through enough and have decided I am worthy of a low-memory, stable VPS
Actually, now that I think about it, I did recently disable an hourly cron-job that had been running. It was a backup script. I wonder if that c-job being disabled, coupled with a server reboot lowered my cache level.
Viewing my graph, I can see that there was a slight rise in memory usage right at midnight, which is when I have my daily (once a day) back-up script running. The level goes down a couple of hours later.
It would stand to reason that an hourly back-up might cause a larger cached memory throughout the day.