when one of my broodies does what you've described
@Rcasady , it's because when she got off the nest to eat and drink etc. another hen went in to lay, so when the broody came back the nest was occupied. Her instinct to resume sitting is strong, so she finds and sits in another nest.
The eggs in the original nest might still be viable, depending on how long they were left exposed and how cold they got, and you can just sit back and see what happens, as you suggest. But studies show that about a week into incubation is a critical time, and from that point of view it couldn't have happened at a worse time.
If you really want her to raise a clutch, I'd start again with fresh fertile eggs asap, and mark them as rosemarythyme said, so you'll know if any other hen comes along and adds to the clutch (you want to avoid a staggered hatch, with eggs started and therefore hatching many days apart). And in this case, if you can, monitor the nest while she's off to ensure no-one else is sitting on it when she comes back, so she can resume on the right nest.