The preauth, which shows up online on your accounts shows the amount initially charged. The tipped amount doesn't get added in until the transaction settles fully to hit the statement. So that's one way.
The person could have had a per-diem that doesn't require receipts up to X amount and they used it all intentionally, so if it's over they would know something is off? That's another way.
They could make this purchase frequently and know how much it usually costs so they don't need the slip to remind them? That's a third way.
They could have left cash for a tip and the bartender already took it so as not to leave cash on the counter when they leave to do something else? There's a fourth possibility.
They might have an app on their phone, or a seperate ledger they keep and just note what they charged down in it, so as not to deal with the clutter of a credit card slip everytim they charge something? Should I go on?.
They have a good memory and can remmber what they paid?

Imagine thinking someone needs the CC slip to know how much they charged. Hoping if the customer complains about the unexpected charge, that your action doesn't get the bartender fired.