Theft from your luggage is unfortunately a thing that can happen. Just like in daily life there are criminals that want to steal what belongs to you. This can surely happen by fellow travelers but also by people who work as luggage handlers in airports, buses or ferry boats. Thailand has seen both arrests of foreign travelers being caught for stealing as well as local employees who work as a luggage handler.
Coconuts Bangkok writes often about this problem. Its like fighting rats; no matter how much you catch of them they will come back.
Phuket airport employee caught stealing Adidas shoes from luggage.
Busted! Baggage handlers at Suvarnabhumi Airport caught stealing from passengers.
Phuket airport staff arrested for stealing from passengers’ luggage.
How people steal from your luggage and how to ovoid theft.