This is a guess, but I suspect you traded currencies (legal tender). If you 'bought cash' when you travelled internationally, and then 'sold cash' when you returned, it is possible that the exchange rate changed during the process. If so, then you may have had a 'gain' or a 'loss' because of the market change. Suppose you bought 100 Canadian dollars when the exchange rate was 1.20Can to 1.00 US. It cost you you 80 US. You returned later and bought US dollars when the exchange rate changed to 1.10Can to 1.00 US. You would get $90 upon your return. You had a gain of $10 because of the exchange rate change. Someone is taxing that gain.
