Can you tell me how safe the backpack you mention. Hav have not owned but but is seems to me that having the backpack on, a thief could get into the backpack without noticing it. (like a pick pocket). Thank you
Could you provide some further information on which Backpack you're referring to???
Any backpack. It just seems that it is on your back and you are unable to make sure someone else could get into it.
You are absolutely correct and that doesn't depend on the type or style of backpack. The issue is how you handle a backpack in a crowded situation. In a crowded bus, subway car, the backpack is off and at my feet in front of me if standing or seating. Use common sense. I do secure all the pockets with small locks but it is how you handle it that is important.
Also, wearing a full backpack on crowded transit can lead to you unintentionally slamming it into another passenger when you turn around.
Important stuff in your money belt where it's safe from pickpockets; sweater, water bottle, guidebook, lunch in your backpack.
A backpack is a good way to carry heavy or bulky things, but not valuables. See also http://www.ricksteves.com/travel-tips/theft-scams.
How "safe" a Backpack is from pickpockets will depend to some extent on the design. I've been using panel-loading Backpacks for years which have lockable zippers, and never had any issues (the pack is always locked when I'm in transit). While the pack could certainly be "slashed", I've never experienced that. I normally also travel with a Daypack and a Camera bag, and both of those are always locked in transit as well.
Top-loading Backpacks are probably also somewhat "theft resistant" as the tops are usually lashed down so not something that can be easily opened. If valuables are stored down towards the middle or bottom with soiled laundry on the top, any potential thieves will get a rude and malodorous awakening if they do gain access to the top of the pack.