Critter dug gravel out from under my barn's cement slab
Unless you have concentrated loads there (commercial vehicle traffic, etc.), don't worry about it. Shovel what you can back in and be happy (assuming that you can plainly see the extent of the excavation).

If it's a concern, Try using some 2-3" pipe or flex tubing. Fill it with coarse sand or fine gravel. Slide the pipe into the end of the hole, then use a pole of slightly smaller diameter to push the material out as you slowly retract the pipe. Use the pole to compact the soil as you work your way out.