I've had broody hens that played the long game. They sit a little longer each day, collect eggs, and continue to roost. Eventually they will begin to sit round the clock. As to how, she may have convenienced her roomies to lay in her chosen spot by depositing her own eggs there for a couple of days.
Are you going to let her sit or break her? There is just nothing like a mama hen and chicks.
I miss having chickies around but dealing with the cockerels is just too stressful for me.
At the moment thankfully no broody girls, but it's an Easter egg hunt these days to find eggs. Now that I have jinxed myself by mentioning 'that which should never be mentioned', I will likely have 20 of them go broody hahaha!