Bill Hilton writes:

Anybody with a brain could also figure out how to open the cannisters and put
explosives in them too ... that's why they swab.

Anybody with a brain could figure out how to put explosives in lots of
things with as small a volume as a film canister, so there is no
particular reason to swab canisters if every other object not being
x-rayed isn't also swabbed in a similar and complete way.

Another option is simply to buy film at your destination.

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