This still happens if I dare to attempt a sneaky lie down while my children are around. I think there must be an atmospheric change, or a signal emitted. They know, they just know.
It’s the same alarm that goes off the second you press ‘call’, about to make a phone call. And when your bum is about to meet the toilet seat. And when you open the biscuit tin.
They don’t need to see or hear anything, they just know, and they come running.