Maybe the maid who cared for the baby had to take an emergency day off.
Or maybe Cecilia Cheung felt that it was not good to keep her seven-month-old son Marcus cooped up at home all the time.
Whatever the reason was, they ventured out of their home.
Which is why Hong Kong paparazzi managed to take the first photographs of the toddler when mother and child were spotted for the first time in public recently.
But Cheung, 39, who has kept mum on the father's identity since she gave birth to her third son in November, had taken precautions to protect her family's privacy.
The actress turned up at her two older sons' school in a car owned by her manager in a bid to evade attention.
Videos posted online showed that she stepped out of the car carrying Marcus, but the footage did not give a clear view of his face.
According to the Sohu portal, Cheung, who was wearing dark sunglasses, showed up for a parents' day in the school with her younger brother Ronnie.
Cheung - whose two other sons, Lucas, 11, and Quintus, nine, are from her previous marriage to actor Nicholas Tse, has yet to share a photo of Marcus on her social media accounts.
It is not known if the father has worked out an arrangement with Cheung to keep the child under the radar. Sharp-eyed observers note that she is not hesitant to share online pictures of what her two older sons are up to.
Ronnie seems to be providing a useful backup for Cheung in case things get out of hand with the media or nosey fans.
In December, she posted photographs of her going for a holiday with her two older sons, together with Ronnie's wife and kids.
While there was no sign of Marcus, a netizen later revealed that there was a woman, carrying a baby, at the airport, and that she was talking to Cheung, raising speculation over whether the baby was Marcus.
But would Cheung, who is devoted to her kids' welfare, be okay with the baby taking a flight not long after his birth?