[throwaway22032]

If you claimed all three of those, you would have to claim all three of them, at which point you would have claimed all three of them, and someone could easily see that.

[bena]

Someone could easily see that if you have an actual reimbursement system in place.

If it's based on the honor system, I could claim the train ticket, then claim a bus ticket and say that the ticket was for something else, you were mistaken. Then claim mileage and say the bus ticket wasn't that trip, you made a mistake. Again, just like the train ticket.

So, yes, the claim needs to be backed by proof of trip or by a deliberate action on your part saying that you are claiming this right here on this date for this trip.

[throwaway22032]

In your example you describe "something else". If there's "something else", then that "something else" is an allowable expense.

[bena]

Lies are not an allowable expense.