Yes, the water should be somewhere in sight in the glass – about 3/4 of the glass after the boiler has cooled and the water has returned from the readiators. You need to check to see if the gauge glass can be drained down to assure that the level in the boiler is also within the level you can see at the glass. Now that dirty water went into the main steam vent, it might need cleaning or replacement. Read the Steam Tips at: http://www.hydronicnetwork.net
Have you had the required yearly service of cleaning the low water cutoff and safety checks done this year?