This happened to my father when he used to work for a road lines company.
(My father while working for a company indulged into inter state goods transportation as a field officer had to travel a lot. He was a frequent traveler to many destinations throughout the country.)
Once he was supposed to reach Mumbai and as usual whenever he travels he makes a list and meticulously checks off each item as he packs his suitcase.
As soon as he got to his hotel, opened his suitcase only to find it had been searched by airport security. How could he tell?
We asked him on telephone he said; On top of his neatly refolded clothes was his list.
Next to each item someone had made a crisp check mark. At the bottom was this note: “you forgot the toothpaste.”