Thieves who target dozing passengers during flights have reportedly come up with a new trick: they take the victim’s valuables and replace them with something that looks or feels similar.

The Hong Kong Police Force’s airport district branch has issued a fresh reminder to those planning to fly during the holiday season that they must keep a close eye on their property while in the air.

In one case highlighted by police, an airborne pickpocket stole a wad of US dollars from a passenger’s backpack and replaced it with a stack of Vietnamese dong notes of similar thickness. One Vietnamese dong is equivalent to US$0.000043.

In another case, a passenger aboard a Hong Kong-bound found that US$5,800 he had in his suitcase had been replaced with Libyan dinars worth just US$480.

The force received 12 complaints from passengers on flights originating from or heading to the city over the first 11 months of 2018, compared with just seven in 2017.

Seven light-fingered passengers have been arrested so far. Most of the suspects are aged between 37 and 50 and hail from mainland China. One has been convicted of theft and sentenced to 12 months in jail.

The modus operandi of offenders is that they steal money and valuables from bags stored in the overhead luggage lockers before take-off. They also take the opportunity to steal items whenever the cabin lights are dimmed during long-haul fights.

The force recommends that passengers check their belongings before touching down so that if they have been targeted, the flight crew has a chance to investigate the matter while the culprit is still on the plane.