Cooling System Burp!
 

Yesterday out of the blue my 2000S "burped" about a quart of coolant out the vent just as I returned from a short drive. Temps were in the 70s and engine temp was normal.
Can find nothing wrong, so I went through stationary bleed last night and a 20 minute drive bleed today. Nothing unusual except I noticed the bleeder valve was being held up in the open position with the system hot, I assume this is normal?
Anything to worry about?

It shouldn't be open (up) in full time operation.
Do a search and you'll find better info than I can provide.