Purses and backpacks go missing at the worst times. Like, when they’re filled with cash, passports, cameras, medications, travel documents, and laptops. Like, when you’re far from home.

Last month, Ron and Sharon Dasil were renting a car in Barcelona. Very experienced savvy travelers, they took all the precautions. Sharon sat with their four suitcases and two backpacks while Ron stood at the counter in the tiny Hertz office near Sants train station. After resisting up-selling efforts by persistent employees, Ron finished the paperwork and asked for driving directions. To make notes, Sharon joined him at the counter for three minutes, max. When she returned to her chair, her backpack was gone.

The office was only about a 12-foot square; the two employees faced out toward the door. When Sharon exclaimed that her bag was gone, one of the employees took a few steps to the door and closed it, pointing to a sign that the Dasils had not seen: “guard your stuff, thieves are around,” or something to that effect.

Had someone been watching from outside, waiting for the office’s only customers to turn their backs? Was it pure flukey timing? In such a small space, why hadn’t the employees noticed the arrival of a new person? Wouldn’t they greet a potential customer? If thieves are around, why was the door propped open?

These are questions the couple is asking the police and the Hertz headquarters. They also wonder if there was some complicity or collusion between the Hertz agents and the thief. “One of them bent under the counter for a while. He could have been texting someone,” Sharon worried.

At about the same time, Paul Hines was checking into the Holiday Inn Kensington Forum Hotel on Cromwell Road in London. He and his wife piled their luggage next to some chairs in the lobby, with their backpack on top. Mrs. Hines sat with the bags while Mr. Hines checked in. Mrs. Hines was briefly distracted when she noticed a man in the lobby with his fly open. That was all it took. The backpack was gone.

Who was the man with the open fly? An intentional distraction? Or just a staff member or hapless guest? Holiday Inn staff claimed to have the theft on video, but wouldn’t reveal much else to the Hines’s. Neither were they very helpful after the incident. They marked the location of the police station on a map, but didn’t get a taxi for Mr. Hines, who had no cash for a cab. He walked there and back again in the rain.

Both the Dasils and the Hines’s lost a lot in their backpacks. Both couples spent considerable holiday time filing reports, canceling credit cards, replacing passports, etc. While the Dasils, frequent travelers, took care of the theft business then got back to their adventure, Mr. Hines still seemed angry and frustrated several days later, when I spoke with him.

Now you see it, now you don’t. We all know to guard our stuff, but it’s worth remembering how quickly these thefts happen, and how frequently. The opportunist thief is lurking, waiting for you to drop your guard. The strategist thief turns your head himself, with some devious distraction or other.

Lobby employees don’t know whose luggage is whose. They don’t know every guest or customer. They are not luggage guards—not even the doormen are.

How exactly are purses and backpacks stolen, right out in the open? The technique usually involves a sport coat or jacket. We know some thieves who use an empty garment bag. The thief simply drapes the cover over the object of his desire and walks off with the goodies hidden underneath—often barely breaking stride. Bob Arno has done this many times on television.

I’ve heard about a hundred too many stories like these. Now I’m calling on you, readers, to help put an end to lobby theft. Watch your stuff. Keep a hand on the heap, or some other body contact with your bags.