Originally Posted by Often1

+1 - While it's always good to do a one-off random audit here and there, if you trust your people so little that you need to resort to this, you have much greater problems than whether they're nicking you here and there.

If you really do want a system which doesn't require you to go looking things up, use a business card which you issue and which you require employees to use for airfare. If someone charges a $2,500 ticket and cashes it in for something cheaper, you will see the refund.