When you find a door closed, perhaps you'll find an open window.
But if the window is closed too, maybe you can see if there's one of those dog doors left carelessly open. And if you're small of hip and shoulder and don't mind crawling on your knees, it would do fine.
Otherwise, there's the mail slot. Maybe. I grew up in a house that didn't have one. And the mailbox really wouldn't serve the same purpose, being as it is, both away from the house and out by the street.
But the mail slot has it's impractical side as well. The most you could hope to slip through a mail slot is your hand. And your hand couldn't even look around on the inside if you are outside wanting in, nor outside if you are inside wanting out. Unless perhaps you have one of those lucite flexible periscope type jobbies that would allow you to look in or out through a mail slot. But ultimately even that would be sort of cosmically cruel. A tease. You could see where you cannot go. Yet.
So you look around yet again.
Chimney? It works fine for Santa. But if legend is to be believed, the guy's an elf. There's that size/mail slot thing again. And here even the lucite periscope thing isn't long enough to tease a view from the roof.
Maybe time to go back to the door. Try the knob this time. See if the door's unlocked.