Radiator overflow tank behavior ?

This was a new one for me. Riding 300+ miles home today from New York. Outside temps in the low 90s and I notice I'm seeing 3 bars on the temp gauge more than normal. I stop and I hear a gurgling sound. A check of the overflow tank shows the level about an inch or more over the full line.

About 3 minutes later I look again and the overflow tank is completely empty. Restart the bike and at speeds less than 50 MPH the gauge is always reading 3 bars. 50 and over it goes back to 2 bars.

I found some wet spots on the left of the crankcase and I assume my top hose is leaking. That's easy enough. I assume the gurgling is from air getting sucked into the hose.

The thing I'm confused about is the disappearing fluid in the overflow tank. Is that expected behavior? I witnessed this on two different stops. Is that just what happens when there is air in the system?