Maybe the maid who took care of the baby had to take an emergency day off.
Or maybe Cecilia Cheung felt it was not good to keep her seven-month-old son Marcus locked up at home all the time.
Whatever the reason, they ventured out of the house.
That's why the Hong Kong paparazzi managed to take the first pictures of the child when mother and child were first seen in public recently.
But Cheung, 39, who has retained her father's identity since she gave birth to her third child in November, took precautions to protect her family's privacy.
The actress appeared at her school of two older children in a car owned by her manager in an attempt to flee from attention.
Videos posted online showed that she got out of the car carrying Marcus, but the footage did not give a clear view of her face.
According to the Sohu portal, Cheung, who wore dark glasses, appeared for Father's Day at school with his younger brother, Ronnie.
Cheung – whose two other children, Lucas, 11, and Quintus, nine, are from their previous marriage to actor Nicholas Tse, has yet to share a photo of Marcus in his social networking accounts.
It is not known whether the father worked out an agreement with Cheung to keep the child off the radar. Watchful observers note that she does not hesitate to share pictures online of what her two older children are doing.
Ronnie seems to be providing a useful backup for Cheung in case things get out of control with media or meddlesome fans.
In December, she posted photos of her going on vacation with her two older children, along with Ronnie's wife and children.
While there was no sign of Marcus, an Internet surfer later revealed that there was a woman carrying a baby at the airport and that she was talking to Cheung, raising speculation about whether the baby was Marcus.
But Cheung, who is dedicated to the well-being of his children, would not mind if the baby took a flight not long after his birth?