The 23-year-old One Direction star was joking about an American burger chain when he appeared to let the cat out of the bag.
“I left my wife and child at home and I was straight out to In-N-Out Burger,” he told the hosts.
The couple welcomed their first child together, baby Bear, in March, but have never confirmed if they’ve tied the knot.
Liam also spoke to Sirius FM about being a dad and revealed he was on nappy duty on his son’s first day in the world..
He said: “I did all the first day’s worth of nappy changes, because [Cheryl] was so tired, she slept more or less.
“All day, there was no poo. I was like this is great, I can handle this.”
Liam also revealed Bear’s middle name – Grey - was a joint decision.
He said: “That was multiple choice, that was together. It was going to be James, because my middle name is James, but I prefer Grey.”
Cheryl is no stranger to a ‘secret’ marriage. The former Girls Aloud star married her ex husband Jean Bernard Fernandez-Versini in 2014 and managed to keep the nuptials a secret from the press for a whole week.
She eventually confirmed the news on Instagram, telling fans: “I usually do not discuss my personal life but to stop the speculation I want to share my happy news. Jean-Bernard and I married on 7/7/14. We are very happy and excited to move forward with our lives together.”
The couple split less than two years later, and finalised their divorce in October last year.