So,
//at start
thisMarker = instance_create(x,y,marker) (CLOSER)
instance_deactivate_object(thisMarker)
thisMarker2 = instance_create(x,y,marker) (FARTHER)
//later
Object(monster){
If(instance_nearest(marker) !=noone){
//object exists
//and it will only return thisMarker2, because it cant see the closer "thisMarker", correct?
}else{
//object does not exist
}
}