They were already on the ground. They picked them up and tossed them in the ditch.
Simple carelessness under the circumstances. Not right. But still plausible under the circumstances.
If you want to hide evidence of your wrongful shoot.
They figured that the smuggler had escaped.
Destroying evidence that you shot, what, 15 times, is okay if the guy escaped? LOL!
The most plausible scenario is that other agents that were following right behind R&C had to have heard at least some of the shots that were fired by the two agents. You can hear ten .40 caliber rounds being fired a mile away out there in the open desert. Easily. Any veteran LEO can immediately recognize the sound of gunshots. The FOS gets arrives there at the scene, immediately asks what's going on and no one says anything about a single shot being fired....??
So they admitted they shot? That they tossed the casings? LOL!
I believe that Ramos' testimony that he heard the other agents talking about gunshots.
What was his testimony about the casings?
But I keep coming back to the sound of all those shots being fired. The FOS arrives on the scene and immediately asks what has happened. One or more of the agents that were following R&C and that were already there says that they heard shots fired. Thus when Ramos is climbing back up the ravine he hears the other agents talking about gunshots. The FOS figures that they're never going to see OAD again so they'll just take the van and its cargo back to the station. R&C's failure to verbally state directly to the supervisor that they had fired their guns wasn't right, but I can still see how such a thing could happen under the circumstances.
If the FOS was aware that shots had been fired (which I believe he was ) then it is incumbent upon him to inquire how the events transpired with this subordinates.