You most likely have a small leak or weap somewhere on your system. If your system is open vented, in that you have a f & e tank in your attic, (this is the small black tank), then the water that weaped from the system is being replaced with fresh oxygenated water, bring air back into the system. Likewise if the system is sealed, it may have a automatic filling loop that will do the same. Radiators will always be the location that traps the air as the are high points across the flow pipework.
Typical locations on a system for weaps are near the boiler, check the automatic air vent and also the pump valves if they are the slotted type. It could also be an expansion vessel failure and water is being released through the safety valve and entering back through the filling loop.
If problem has always happened, then it could be a badly designed system, in that it has been installed in negative pressure drawing in air through the open safety vent pipe or the system could be pitching driving water out through the osvp and back in through the expansion.