This is a true story, although I won't blame you if you don't believe it. I once lived with a cat called Geraldine (aka Gerry Howl). Geraldine got into the habit of scratching on my bedroom door every morning until I woke up to feed her. In the afternoon she would stretch out on my bed, the only place in this apartment that got any natural light. One afternoon, as I saw her sleeping lazily in the sun, I started scraping on the door with the tips of my fingers. I did it until she woke, and looked up at me with (what seemed) a vexed expression. I smiled back, but kept it up for another minute or so. And, I kid you not, she never scratched on the door thereafter.
It must be frustrating to be an animal living in a world of doors (or cans of food for that matter) that you can not open. Pico has to wait until (dum) someone (aliquis) entering (intrans - another participle!) opens the door. Then he secretly (clam) follows her.
Pico waits outside the restaurant until someone entering opens the door. At last (tandem) a certain woman enters. Pico secretly follows her.