overheat but no excessive pressure

[It'sNotWorthComplaining!]

just checking if a Disco overheats does the expansion tank cap necessarily have to release the coolant?
Could an airlock cause an overheat and not pressurize the system to cause an overflow through the cap?
Asking as some one has apparently fitted a new radiator and head gaskets and water pump and fitted a new cap and the vehicle overheated in the hot days just gone, but the cap did not release any coolant

[bee utey]

No good reason for coolant to escape so long as the cold coolant level was at the star mark 1/2 way up the tank. Unless it's boiling its heads off.