So you found a misload in your car.
You didn't sheet it up when you found it.
You didn't sheet it up at the next stop, either.
Or the next stop. Or the next...
You finished your entire day, then drove back to the building, without sheeting it up. At which point, your supervisor confronted you with the fact you didn't sheet it up.
To believe this was just a mistake on your part, we would have to accept that there was a compelling reason for you not to sheet it when you first found it, and that you just "forgot" about it throughout the entire rest of your day, and at no point did you see or handle the package in your truck for the entire rest of your day to "remind" you that it existed.
To make matters worse, we know this was a SALT package. You were almost certainly SALTed because of a suspected issue with your behavior (either dishonesty, or ability to follow methods/instructions).
Dishonesty is the most plausible explanation for your behavior. This is why you were fired. It's just like if you were at a store and put some merchandise in your front pocket along with your phone, and later walked out of the store without paying for it. Oh, and you took your phone out of said pocket a dozen times after you put the merchandise in there, but uhh, didn't notice the merchandise that shouldn't be there during any of those times somehow. Maybe you totally forgot it was there and never noticed it again, but good luck convincing the police, a judge, or an unbiased jury that you didn't intend to steal it given your actions were 100% consistent with intentional theft and any reasonable person could have been expected to (1) not put the merchandise in there in the first place and also (2) realize it's in there and not steal it before walking out of the store.