It's not so uncommon that you don't need to be prepared, and take steps to counter it.
The really bad news is that inflight theft appears to be on the rise.
Passengers on an aircraft are virtually walking safes, with the door wide open.
They might be travelling with laptops, mobile phones, expensive jewellery, cameras, tablets, passports and cash-stuffed wallets.
In many cases they're only lightly guarded, but a few simple steps will make you a harder target.
Stow your carry-on bag in an overhead locker where you can see it. A carry-on bag you can lock is ideal. If it doesn't lock, place it in the overhead bin with the zipper pulls on the bottom, a thief won't risk taking a bag out to get at what's inside.
If you're planning to sleep, put your valuables inside your carry-on, not in the seat pocket or in a bag on the floor.
When you use the bathroom, take your valuables with you, or put them into a high-security bag such as a Travelsafe from Pacsafe, which is reinforced with steel mesh and comes with a cable-type lock.
If something seems to be missing don't panic. Things slide around during a flight, phones can fall out of pockets and get swallowed up in seats.
Ask cabin crew to assist if you need to, but above all, be alert for suspicious behaviour, and not just from fellow passengers.
From time to time, air crew have been caught with goods pilfered from passengers.