Was driving home the other night from work and my gauge cluster said "check gauges". My temp gauge was on hot. Pulled over shut truck off and popped the hood. coolant was low, no big deal. Start truck back up temp gauge is at normal all the way back home 20 miles later. Next afternoon filled up the res with coolant, truck ran fine. Coming home at 12pm same thing happens. pulled over, shut truck off then turn it back on, and it's normal. So where is my coolant going? It's a good inch on the res low each time. Engine oil is normal in color. Seems like If I keep the res on the low side everything is fine. But once I fill it to normal then I get check gauge beep. I replaced the radiator last spring.
I'm lost and confused.

 

I'd suggest pressure resting the cooling system...