The amount of drugs needed to make smuggling worthwhile is pretty significant — as in, you’d-notice-it significant. Drug mules are cheap and disposable so doing anything elaborate isn’t really worth it. As long as you the innocent traveler keep your bags zipped up and locked and you don’t accept unknown packages, you should be good. Remember, trying to slip something surreptitiously into someone’s bag before customs and then retrieve it safely after customs is very high risk for little reward when a mule is cheaper, can carry more, and won’t go straight to the police (or trash can) if they discover a strange package in their backpack.
The way “innocent travelers” usually get caught up is that they travel to some drug source country for holidays and while there, they strike up a friendship or romance with a local.
On return, the local asks them to bring something back to their home country — a present for some family there. At the airport, they are given a suitcase or roll-on full of “presents” but told not to look inside so as to not spoil the fun.
At customs, “fun” ensues.
Especially if it’s at Dubai or Singapore, where their security agents and judiciary are especially “fun” people.