Diddy
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It depends on where you live, ie in the cities most people live in apartments and all apartments have peepholes on their front doors. The intercom is usually audiovisual too so most of the time you already know who's there. Am trying to remember if I've seen one in the UK, probably not, I remember there was that opening for the letters on the door that could have served the purpose, but it wasn't at the right height.
Yeah using the letterbox wouldn’t be very discreet but at least you might see if they’ve got a big package for you