Hello and welcome to JustAnswer.
I'm very sorry to hear about your situation and hope I can help.
If you can get a statement or an affidavit from your coworker that confirms that you did immediately ask about the keys and your manager actually had them at that point and then hid them, that would definitely benefit your case. It's even better to have him present at the hearing to support you. That would change the situation from a he said/she said situation giving you unbiased support of your version of the events and would go in favor of you being terminated through no fault of your own rather than through cause.
Your employer appears to be arguing that you broke a reasonable employer rule by failing to lock up your computer and drawer in the past and misplacing your keys. The employer would need to prove that this was a reasonable rule and that you understood it to be one. (Normally this means that they would need written documentation in an employee handbook or proof that you had been advised).
The primary thing that would be working against you is that you had been written up in the past for not locking up your computer and your drawer, so it's likely you would have been aware of some rule regarding keys.
Also, if there's any reason to suspect that your manager treated you differently due to discrimination based on a protected characteristic (such as sex, race, national origin, or ethnicity) that would also significantly help your case.
I hope you found the above information helpful.
Please don't hesitate to ask me any follow up or additional questions, as I want to be sure you're as prepared as possible for your hearing.
Thanks and best of luck!