Magic coolant loss?

[kaoskingnd]

Today I almost got all the filters replaced and will flush all the systems this weekend, but I am loosing coolant fluid really fast and I have checked every hose and connection. I loose about half the res every three or four days, but it does not show up anywhere. Plugs are fine no coolant discoloration. I keep hearing about a mono-valve could this be a culprit. I am completly stumped.