if (for example) the system is gradually drawing air in when running, and it is gathered in an unused/occasionally used radiator or pipe spur, the symptoms you would see could well be
a) when the system is running 'normally' there might be a slow dribble through the overflow ( corresponding roughly to the rate at which cold air is drawn into the system; there would only be a few hot bubbles at any one time)
b) when you open the valves to normally unused radiators (which may have gathered gas in the meantime, either inside or in the pipe spur leading to each one), there will be a sudden expansion in the gas (it will go from cold to hot) and this will push water out of the overflow.
If the system is merrily corroding internally, gas will be generated continuously and may give similar symptoms. Corrosion may produce hydrogen in which case the bleed gas from the radiators will certainly smell different and may be inflammable.
The system will draw air in if the pressure is low enough at the pump entry. If you can do, fit a 'T' near the pump entry and (temporarily) fit a clear plastic hose to this, configured as a second overflow. This will also operate as a pressure indicator (manometer) at the pump entry. If there is enough resistance in the pipework (which will vary with the number of radiators that are working at any one time) you may find that the pressure reduction at the pump entry exceeds the static head of water that is pressurising the system. This is quite likely to occur with an upstairs pump, and if it does the system may be drawing in air eg via the cold feed, auto bleeder, etc.
cheers