ex_crt_effect == 1 is often the case with ordinary citizen. According to my tests, SEE_CREATURE is called after LookNeighbor calls from AI (such calls include NPC and PC), or conditional scanning like LookNeighbor(simple scan around space), from deep parent AI (in this case, only PCs are detected). It is possible that ex_crt_effect == 1 allows you to ignore any kind of invisibility when checking LookNeighbor and at the same time the NPC receives exactly SEE_CREATURE with subsequent transitions on super-calls.