They do sneak off the nest when nobody's likely to be around to quickly gobble some food and get a drink. If you want reassurance, you can leave a bit of food and water nearby but the other hens may just take it.
I'd mark the eggs she's on with a pencil, because the other hens may add eggs, which won't hatch in time and may mean some of the good ones wind up out in the cold for too long, lowering your hatch success.
Congrats on the free chicks!
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