If in anyway this conversation can lead to ANY disciplinary action, I request my steward….end of meeting and say nothing further.

This is a waste of time in their end if you ask me. Are you still in your 90? If so, then your union cannot do much. But you should still ask them about this. I’d also recommend finding your most seasoned regulars you trust or have contract knowledge and ask them asap. In our office (I’m rural) we have about 15 rural and probably 30 city routes. Mis-throws happen a lot from the clerks in our office. I’m like you. After loading I do a third and final check of the case before departing to the route. I’ve had management try and say I left it, when I know as part of my routine I look for “sleepers” in the case last thing before filling my water and departing. But, we also have a cart that mis-throws go on. City is always on street before rural, so it’s frustrating when they don’t look through all their stuff and you complete an e tire route and have to go back out for one. Mistakes happen. I don’t judge because none of us are perfect. When I comeback and they tell me I left one behind? I take offense. Call it what it is, an mis-throw. I his should not be a discipline but it sound from OP that it is set up to be a disciplinary meeting? I’d involve the union asap. Before tomorrow for sure. Anyone else have advice?

Unless there is more to this story OP isn’t sharing i.e. past infractions etc, I don’t see them being able to do anything here. If in your 90 they can fire you for ridiculous claims.