For those asking what the difference between leaving an animal 8-12 hours overnight and for a few days..yes, it's unreasonable to expect every single owner to check their animals every few hours around the clock. I personally need at least 6-8 hours of sleep a night to function and will very rarely be able to wake up in the night if I set an alarm. But I'd never leave my animals for a few days without someone to check on them.
Yes, bunnies can drop ill any time - Joey died the other day after being fine the very same morning. However, there's the fact that a few hours can make a big difference, and the off-chance of passing by the bunnies when one begins to get seriously ill gives them a much better chance of survival than if you're not there at all.
For example, say a bun developed GI stasis overnight. If you home, it may be 8 or so hours before you notice, but there's a high chance that you'll check them in the morning and discover it, and be able to rush it to the vet for treatment. If it developed the exact same problem, the night you'd just left to go away for 2 or 3 days, it will be left for a lot longer and there's a much greater chance of it not making it as it's not got that treatment.
Yes, it may be fine, but I'm a big believer in avoiding "maybe" circumstances. Some buns may be fine if left in a non-fox-proofed run, but those that have been caught by foxes are enough to deter me from trying it. Some female buns may be fine and not get uterine cancer if they're not spayed, but I'd like to give Nina all the chances she can get.