The rectangular hole and strap hold the first aid kit required for Japanese domestic sales. Or so was said in the very old days of Prius online discussions.

What Bob discusses sounds like the 'tow hook' stored by the spare tire, and intended to be screwed into the mysterious hole in the the front bumper (vehicle left side). By now those front-facing screw holes are all filled up with road grime (except those blocked with wine corks or similar). The new model Prius has a plastic cover over its tow-hole.

DAS