As stated by the other poster, hens regularly come off the nest once a day for about 20 minutes to eat and relieve. In warm weather, they can stay off for about an hour. Cooler weather they tend to make short trips.
Confusion with a new broody often does cause them to not return to the same nest. That is one reason having a brooding in her own hutch/pen prevents these kind of mishaps.
Eggs are surprisingly hardy, and while hatch rate will often suffer, being chilled is not the end of everything. Put her back on the nest and then preferably rig up some sort of fencing that prevents her from getting confused as to what nest she should return.
If a hen is consistently leaving eggs for portions of the day, she is not really devoted to the brood. Commercial hens often semi-brood, or "sulk" as their genetics have been tinkered with over the years for egg production and not brooding hormones. I have seen commercial line birds fake a brood for a bit but not stay the course.
It really varies with the hen, and having isolation for the hen from interruptions (others pushing her off the nest) and confusion when she's in the yard can definitely increase your likelihood of success.
Let us know how it is going. Good luck with your new brood

LofMc