Allows hiding mobs/areas from observers without completely forbidding

Basically for more common usage for place we would prefer people couldn't peek into before people are there ICly, like gateways.

code/modules/mob/dead/observer/observer.dm

@@ -341,6 +341,8 @@ This is the proc mobs get to turn into a ghost. Forked from ghostize due to comp
 		var/area/A = areas[key]
 		if(A.z in using_map?.secret_levels)
 			areas -= key
+		if(A.z in using_map?.hidden_levels)
+			areas -= key
 
 	return areas
 
@@ -353,7 +355,9 @@ This is the proc mobs get to turn into a ghost. Forked from ghostize due to comp
 		var/mobz = get_z(mobs[key])
 		if(mobz in using_map?.secret_levels)
 			mobs -= key
-	
+		if(mobz in using_map?.hidden_levels)
+			mobs -= key
+
 	return mobs
 
 /mob/observer/dead/verb/dead_tele(areaname as null|anything in jumpable_areas())