Passive Defence can mean any form of non-combatant defence, from buildings being planned and built that in plan view take on several different directions (like a barrack block with an 'H'-shaped plan-form) or huts aligned along headgrows for concealment, to various forms of camoulflage such as hedge painting to disguise runways, or shadow generators forming roofs of buildings to create a shadow of a tree canopy etc. The list is endless.
Barrage balloon sites are not passive, as the balloon cables were designed to bring down enemy aircraft (hardly passive), where as a decoy site is passive as it diverted an enemy aircraft away from its intended target and created the illusion that the bomb aimer and pilot had found their target. If however there was a Bofors gun in the field opposite to bring the enemy aircraft down then that is not passive.
Dare I also say that after 1942 and generally speaking (so not in all instances) a lot pillboxes took on a more passive role as they were abandoned as fixed defences and were replaced with more mobile defences and defence localities. The pillbox being used as a dummy pillbox.