That's a Two-fer

Easiest in Green Hill Act 1. When the act ends, the Goal Plate drops from the sky - jump into it and knock it around in the air before it lands, and it spawns a hidden item monitor at a fixed point. GHZ Act 1 has two such points (between the purple flowers on the left, and under the yellow sunflower on the right), so bat the plate about and a single ending can uncover both boxes.