During construction, my basement was roughed-in for a bathroom (tub-drain and toilet flange). The tub-drain was stubbed-out below grade, and before the pour, the contracter installed a plastic box (1' x 1'), extending into the gravel substrate, to prevent the drain pipe from being cemented-in.
Now that I'm finishing that basement, we decided to install a shower stall instead of a tub, so I had to cut some of the concrete away, to accommodate the drain position in the shower pan. In doing so, I've exposed more of the gravel substrate, and with "April Showers", that gravel is moist!
As long as my sump pump (and backup sump pump) don't fail, I can't think of any reason to do anything special with this hole (just fill-in with pea gravel to keep everything in it's place) Is there any compelling reason to do anything else? I thought I might use some expanding foam sealant, to fill-in the bottom of the hole (and also seal-it), then cover with the gravel.