What do i do if someone knock on my door and i couldent see them?

ok someone was knock at the door i looked to see who it was i couldn't see anyone and so i turned on the lights and there was no one there then the same thing happend to my nabor across the street and they truned there lights on and no one was there and thishappenedd all though out the block was someone playing ding dong ditch or is this a Sirius problem