Why does my cat meow in front of the door?
I have recently returned from a trip and I've found that sometimes my cat goes to the main door of the house and starts meowing. The cat didn't use to do this before my trip. During this trip I left the cat alone at home and a friend of mine came everyday to feed the cat, play a bit with him, ...
Why does he do that? Is it normal? Is there any way to stop him from doing this so insistently?
Cats meow mostly for one reason, to communicate with humans. Feral cats usually don't meow. Also, meowing is akin to a baby crying to its mother. Domesticated cats retain this behavior with us, since we're their caregivers.
What I'm hinting at is that your cat is most likely asking for something from humans. If they look at you while meowing, or are near you, it's a message for you. Otherwise, your cat might be "calling" for your friend.
Maybe your friend let the cat outside accidentally and they want to get back outside?
Cats can actually be very socially oriented and miss people when they're gone, despite what pop culture would have you believe. I personally never leave my cat alone for more than a few hours. If I have to leave town, I leave him at a family member's house. That's just me though.
Maybe your cat is worried you'll leave again?
My suggestion: find something to lure your cat away when they do that. Maybe a toy or something you know distracts them, not a treat / reward. Don't summon them specifically. We don't want your cat to think they'll get attention for doing this.
When they're not whining, maybe play with your cat / give them treats by the door. This way we'll replace whatever bad memories there might be with good ones.
It looks like your cat wants to go out (which probably is what the caregiver got him used to in your absence). My cat usually stands at the door and meows when he wants to be let out. The balcony to my apartment is open breezy and with lots of birds flying around. This is great entertainment for cats and he loves being there.