So let's play 'find the yellow shopping bag'.
In the photos of Dec 18 it shows up 3 times:
019.jpg, 020.jpg it is on the other side of the room being used to cover the main blood spill so the camera stand isn't sitting in Meredith's blood. These were taken at about 2:45pm. The clasp is photographed on the floor next to the lamp cord at 3:52pm. at 4:01pm Meredith's bloody sweatshirt has been dropped on top of the lamp cord seen in 064.jpg. And in the wider view of 065.jpg we see the shopping bag at the back corner under the desk.
In the video of Dec 18, the bag shows up first in the bottom right corner of the frame at 02:49in front of Meredith's desk. And again at 06:26, and 28:29.
At time index 37:29, the camera man pulls the junk out from inside the desk and the camera pans back to see the shopping bag and the first view of the white spot. At 37:40 there is a very clear view and this is definitely not the bra clasp. The frame at time index 38:10 matches the above photo.
At index 40:15 the shopping bag is being placed under the camera stand.
In part 2 of the video at index 00:13:41, the shopping bag is on top of the debris pile in front of the desk. At 00:34:26, most of the debris has been moved away and the clasp is "discovered" on the bare floor.