Unless you have a web cam on her and watch it constantly throughout the hours of daylight, then I cannot see how anyone can say she didn't get off the nest. You just worry that she didn't get off and convince yourself of it. Broody hens are supposed to be sneaky. I've seen mine making a detour back to her nest when she knew I was watching to try to fool me about where her nest was. These are natural survival instincts and even though modern day hens have lost a lot of their wild ancestors behaviour, broodiness is one aspect where we get to witness it which is why it is such a thrill to watch them.