i have seen termites enter a house through a plumbing leave-out such as this. Other bugs/vermin? If it's a fiberglass pan, it's best to set it in a bed of mortar. so it might be best to fill the hole anyway.

Drill into the slab at the mid-line in half a dozen places so you can epoxy in a few short pieces of rebar. Fill the hole with sand to the bottom of the existing slab. Place concrete and level. The rebar will prevent possible movement or setting of the patch in the future.