My DH occasionally uses the RS "Hidden Pocket" to carry valuables. (Here's a link if you don't know what they are:) https://store.ricksteves.com/shop/p/hidden-pocket
They're very clever; your belt goes through a loop (2 colors, light and dark) to secure the "pocket," then you tuck the pocket inside your pants. Great idea, right? Well, sometimes...
On our recent RS 20 Day BOE tour, my DH wore the hidden pocket on the flight over to Europe, planning to switch to the less convenient money belt when we arrived at our first hotel. But when we arrived at our Haarlem hotel, he discovered to his horror that the pocket was gone! Two credit cards, one debit card, and half our emergency cash gone!
We were able to piece together what had happened; it was an unfortunate confluence of events. Our plane to Heathrow was late, leaving us about 20 minutes to go through security and make our way to our connecting flight to Schiphol. I zipped right through security, but he was held up. The agent asked him to remove the shirt he was wearing over a tee, and also remove his belt. As DH was unfastening his belt, the buckle broke. So there he is, the buckle in pieces, the shirt tossed on a pile of luggage, the security agent tapping his toe, and the clock ticking. DH whipped the belt off, got checked again, then grabbed everything an made a dash to join me. He threw on his shirt, cursed the broken belt but put it back on as best he could, and we raced off to our next gate, barely making it in time.
Our hypothesis is when he whipped off the belt, the hidden pocket quietly slithered down his pants leg, evidently wanting to start life anew in London.
The folks at our hotel (shout out to the Amadeus in Haarlem!) were very helpful; we were able to cancel three credit cards and one debit card (turns out he only had two credit cards, but we weren't sure which one he had left at home, so we had to cancel all three possibles.) Bummer, because we were left with only one credit card and one debit card. Mine. So I, of course, spent the next four weeks being afraid I would lose them. And then it turned out my debit card had some problems, but that's another story.
The moral of our tale? Slow down. Take inventory at every stop. No, don't pull out your money belt or bra pocket or whatever you use, but assure yourself all is well before you leave security, immigration, passport control, the bathroom... It would have been a lot less stressful had we resigned ourselves to having to take a later plane out of Heathrow, but of course had we done that we wouldn't have had any idea of what we had avoided.