"the gun was left by a veteran member of the SWAT team who forgot the weapon in the grass while packing up gear after the standoff ended."
In the Army, one of the worst examples of neglect you can commit is losing a weapon. Essentially, there is no good reason to lose a weapon if you are concious. I have seen entire bases locked down for one missing rifle. When the problem reaches that level of command, at least one person's career is shot.
That's not to say I don't feel sympathy for this guy. I've had a rifle stolen from under my care many years ago by OPFOR during field exercises. I spent a good portion of the duty shift trying not to vomit from nausea after finding it missing, until a grinning Specialist handed it back to me in the morning, having been highly praised for infiltrating the center of camp and bringing back with my rifle. I can say that never happened to me again. I'd guess if this member of the SWAT team was in the same state when he found out, and it probably will have the same effect on him: if it's not in my hands, it's on my back; if it's not on my back, it's locked up.
What needs to be done, besides some degree of punishment to make a point, is a review of how SWAT teams manage accountability for sensitive equipment. This wasn't just an individual failure. Where was the team leader when it was time to leave the scene? Why didn't he do an equipment survey before leaving? How often are sensitive items accounted for? It speaks volumes about security practices and leader responsibility when it's a homeowner calling in to report a missing police weapon. There should be more than one person receiving punishment in this case.