Mickey is very smoochy and starts meowing as soon as he gets through the cat flap. But I don't think anybody told him to keep his meows inside.
He wanders around the flower garden or sits on the front porch and meows just like he does inside. Sometimes, he sits in the flowerbed and meows--maybe, he's singing his favorite song! There's always a hint of urgency in his meow but he doesn't seem anxious. It's very strange to see and hear.
I don't think he is upset or searching for something. I believe he just like to meow!