Sorry Mark - I thought it was clear that tthe first link applied to the AIRLINES - but it does have some insights into how the IRS see vouchers/ compensation in general
https://www.irs.gov/publications/p525/ar02.html seems to list all sorts of thing you might have as income but they don't list repayments from airlines. A google search doesn't bring up anything either
my guess is that it's such a small amount overall it hasn't come to their notice - they might try and get you under "any other Income" but unless you make a fuss I doubt they'll care - it can't be a vast amount anyway?