Hasn't anyone here ever broken a dish? A pot or something slipped out of someone's hands onto a pile of glass plates and there was breakage. When glass breaks, it can fly all over the place; if you have EVER had to sweep it up, you know that. A couple of small shards/splinters of glass could easily have landed in a prepared sandwich or in the sauerkraut, or whatever, and gone undetected.
I'm not saying that IS what happened, but there is as much reason to believe that as there is to believe the glass was deliberately broken and planted in the cop's sandwich.
If it was deliberate, I hope they treat it as attempted murder, or aggravated assault at least, because it could easily have been fatal if he had swallowed it.