Tintin is bored.
This is not uncommon. The orphanage ("it's a group home," the nuns would chide him, but that's semantics) is boring. Exploring the building was fun when he was six, when he was eight, even sometimes when he was ten. But now he's thirteen, and he knows every nook and cranny, and he's bored. So he's hiding in the vents again, sitting above the Mother Superior's office. She has a meeting with somebody today. If the meeting is interesting, he'll listen in. If not, well, he's got a comic book and a flashlight.