I don't even know what Aloha is (besides a Hawaiian greeting), but you could go to the basics...
[ul]
[li]Get a security camera and compare physical actions to receipts in the system. (I'm kind of surprised if you don't have that available already.)[/li]
[li]Have someone come in undercover (unknown to the servers) and order some very specific things, pay with cash, and demand a receipt. See if that charge comes through.[/li]
[li]Does it correspond to just one (or two) people on the schedule? If it only happens when "Joe Pockets" is working, you might keep a closer eye on him.[/li]
[/ul]
Assuming your people aren't stealing...
[ul]
[li]Does Aloha have any kind of error logging. Maybe something is wrong that kicks in when the clock rolls over at midnight.[/li]
[li]Go do a charge yourself at about 2am. See if it goes through. Maybe it gets queued for the next day due to the credit card company doing nightly maintenance or something (just guessing).[/li]
[li]Do you have an employee coming on at midnight that maybe needs some training? Maybe he's just doing it wrong.[/li]
[/ul]
You mention that it has happened on "multiple occasions". Does that mean sometimes it doesn't happen, and you do see charges? Or is it pretty consistent?
Final thought, if you have someone named "Tammy S." working that late shift, it's her. No question about it. I used to date her and she's really crooked.