I have one around the same age that did that except she was roosting at night. It kind of started with staying in the nest box a little too long after she'd laid but not every day, as if she was toying with the idea of going broody but had not yet fully committed to it. Then she finally did go full blown broody today with breast feathers pulled, growling and that broody clucking.
She clearly had plans to stay in the nest box tonight so I put her in the breaker at bed time. I think there's a wide spectrum of broodiness, probably more so with pullets who are new to it all.