[SlalomStallone]

You can just pop open a section of the zipper, then use the zipper over the opened chunk of zipper and it re-seals it

https://youtu.be/zMTXzQ0Vqn8?t=111

[tcdent]

Ah, if it has two independent pull-tabs and they are locked together this works.

If you have one pull tab locked to the end of the zipper this does not.

[Zombieball]

Come to think of it some of my luggage also has locks that secure the zippers in place. Similar to this photo:

http://www.airlineintl.com/files/15/products15988.jpg

This would also hinder this method of opening a bag.

[IE6]

Open - probably not. Open and reseal and you are none the wiser - probably.