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I have always thought a thief would just take the whole bag.
2) deter someone quickly placing "something" in my bag temporarily.

Would not call it an urban myth at all.Actually, I have tried the biro trick, it's no picnic to open the case. Urban myth kind of, at least as far as timing....
If you have enough practice:Indeed, I would think that required a bit more planning and experience with this sort of thing. Really not much can be done to prevent someone with a plan.
If you have enough practice:
Open A Locked Suitcase Without Leaving [much of] A Trace | Lifehacker Australia
 , but on occasions have not bothered. I have travelled to NZ and ADL this year without locking checked luggage, and although I had a lock at the time, did not even bother in the LOTFAP last year. Nothing I cannot afford to lose goes into the checked luggage, locked or otherwise. Storyteller makes a good point though...in hotel rooms, if a safe is not available my valuables do go into the large case which is then locked. Not perfect, but it does in the circumstances, and if I say carry my passport on my person, I reckon I've got more chance of losing it that way than having it knocked off from my locked case in my hotel room.
, but on occasions have not bothered. I have travelled to NZ and ADL this year without locking checked luggage, and although I had a lock at the time, did not even bother in the LOTFAP last year. Nothing I cannot afford to lose goes into the checked luggage, locked or otherwise. Storyteller makes a good point though...in hotel rooms, if a safe is not available my valuables do go into the large case which is then locked. Not perfect, but it does in the circumstances, and if I say carry my passport on my person, I reckon I've got more chance of losing it that way than having it knocked off from my locked case in my hotel room.
