You usually don’t expect your possessions to be stolen when you’re stuck in an airplane. After all, where would the thief go if they decided to make off with your stuff in the air?

But, these things do happen, as a few passengers on one South African Airways flight found out.

According to the Telegraph, two passengers demanded to call the police when they found that some of their valuables had gone missing during a 13-hour flight from Johannesburg to Hong Kong last week.

South African Airways spokesperson, Tlali Tlali, told the Telegraph that one of the passengers who made the complaint noted that they saw some fellow passengers “who had behaved in a suspicious manner” and that they had been seen “opening some overhead compartments while other passengers were sleeping.”

But when police investigated the suspects, nothing was found, according to the Telegraph. TravelMole reported that the objects that the passengers thought to be stolen were later found by cleaning staff and returned to their rightful owners.

Unfortunately, in-air thefts can happen to anyone, especially on long flights when no one is paying attention. Absolute Software, a tracking and recovery solutions company for electronics, claimed that in 2012, "24% of travel-related thefts reported to the company took place up in the air," reported CNN.

Specifically on flights to-and-from Hong Kong, there have been numerous reports of people having cash stolen from their bags. One common tactic is people replacing larger bills found in a bag with smaller denominations, so it doesn't seem like anything is actually missing, reported the South China Morning Post.

This isn't the first time passengers have reported thefts on South African Airways, either. In 2016, a passenger claimed that $1,800 worth of cash and jewelry was stolen on a similar flight from Johannesburg to Hong Kong, reported the BBC.

The best way to protect yourself, of course, is to be alert and make sure you’re aware of your belongings. One easy way to do this is by placing your bag in the overhead bin across the aisle from you instead of above you, pulling all the zippers to one side and placing that end at the bag of the storage compartment.

It’s also helpful to keep any valuables you don’t need at home, and your most important items closer to you in your personal bag below the seat. At the very least, it’s important not to rummage through your stuff or make a big show of anything valuable you have.

One great way to protect yourself is to invest in a good bag that comes equipped with anti-theft features.