The mother octopus never left at all. It didn't move until the eggs hatched. The "checking in" was done by the scientists ("they"), on the octopus ("it"). Not by the octopus ("they"?) on the eggs ("it"... cannot be used to refer to 160 eggs).
Your confusion may stem from the fact that you mis-read thaumasiotes's first comment and you were on the back foot from that point forwards.