What word is better than a "Danger Word"?

In this story, there was a word exclusively known between Grandpa Joe and Kendrick. This word was to be used to signal that the coast was clear, that it was safe to come out. The problem in this story is that seemingly normal words can still come from "turned" people. this was the case for when joe and kendrick went to mike's store and mike looked alright, it was just that when he talked he seemed a little off. also, in the moment before joe had shot cassie, he heard her say, "daddy". where exactly is the line that we draw to distinguish between the affected and the unaffected? of course blood on their clothing and in their mouths are clear indicators that they will probably eat you, but what about the hitch hiker that joe and kendrick saw, and  mike's three sons running after them? those were all people that they had a relationship with, whether as friends or family. i feel like it is that familiarity that impaired joe's and kendrick's judgements on whether to shoot them or not. the moment something is familiar, they stop and think, and thinking takes up valuable time that coulve been spent running, or mobilizing themselves to defend from getting bitten. in the very end of the story, kendrick sees grandpa joe, who says their safeword "breakfast" we are left to wonder if it is realy grandpa or not, because joe did say earlier that the safeword no longer applied, because the "turned" can do anything that enables them to obtain their food. i feel like the safe word here is used as a proclamation of having found a meal, rather than it being safe for kendrick to embrace his grandpa agan.