Depending on the countries and/or your destination ….

Recommend to use luggage, including carry on luggage to have fewer pockets and zips as possible. Even consider a hard case luggage this will prevent people inserting items into your luggage or removing items when you are not paying attention (distracted) or when the luggage is out of your control.

Back in 2015 the Philippines International Airport in Manila (NAIA) made worldwide news with the reports of bullets being planted on unsuspecting passengers by airport workers. Passenger would go through X-ray machine and the bullet (generally only one..) would show up… Can face big fines and long stay in prison for carrying a bullet through the airport and onto aircraft. So personnel would then offer you to pay a bribe so you would not be arrested (if you did not pay, they did arrest you and charged). The locals called this Tanim Bala.

News articles:

Philippine airport bullet scam putting travellers on guard

Passengers targeted in a ‘bullet scam’ by staff at Manila airport

Wikipedia – 2015 Ninoy Aquino International Airport bullet planting scandal

You can see (this is one of many) how it is easy for people to get into your luggage.