I have a simple technique that I use in situations where I must be separated from my rucksack. I obtained and carry with me in my rucksack at all times, A plastic, Nite-Ize "S-Biner" in size # 6. It weighs 45 grams (1.6 ounces). It measures 14.5 cm long by 6.5 cm wide and 1 cm thick. Here is the web page:
http://www.niteize.com/product/S-Biner-Plastic.asp (NOTE: they are available in larger sizes.)
The size #6 S-Biner that I use has a weight rating of 50 pounds (23 kg). My pack, even when I was very new to the Camino and stupid, was never nearly that heavy. I started at 32 pounds (15.5 kg) last year and this year am finally down to about 11.5 kg. I am a big guy, and all my stuff is concomitantly larger and heavier.
Originally, I bought the S-Biner to hold my rucksack off the floor at albergues when the bunk beds had pipe-like frames to avoid water, dirt and insects. It works brilliantly at this. The two spring loaded "gates" open to 2.5 cm or about 1 inch.
Then I discovered, and am sharing here, a much better use. When I stop for coffee at a cafe, or line-up my rucksack outside an albergue if needed, I simply and discreetly clip my rucksack to something else; a chair, fence, pole, a pair of hiking sticks, etc.. Even an adjacent rucksack is adequate. I would rather apologize for accidentally clipping my bag to someone else's rucksack than have to replace all my gear. I should note that it also benefits the other person's stuff as well.
I always do the same thing on a bus or a train. If I cannot clip my bag to the luggage rack, or a structural piece of the bus's luggage hold, I just clip it to someone else's bag.
This way, if a thief tries to grab MY bag and make a run for it, he is grabbing two bags, or is attached to a solid object. This will make it virtually impossible for them to make a discreet or fast getaway. This makes it easier for them to be stopped or apprehended before they can figure out what is wrong.
Grabbing and running with 12 kg is one thing. But suddenly finding yourself carry twice that weight, and dragging it is not exactly what the thief is looking for. My simple "S" hook clip prevents a clean and quick getaway.
My S-Biner clip is in "lime green" as it most closely approximates my rucksack color. This S-Biner is available in about six colors. If you opt for a color that "blends" with your rucksack's prevalent color, a thief will not notice it as readily.
Yes, it is admittedly low-tech and not very secure in the sense of not being an elaborate lock. But I was was trying to find a reasonable solution that would be useful for more than one purpose. A carabiner, regardless of size, can always be used for multiple things. I was also trying to keep the weight down - as always.
Along the way, in two complete
Camino Frances, I have also found that this S-Biner works as part of my expedient clothesline system, in an albergue or outside. it is very good at hanging anything fairly heavy or solid off a floor like wet boots, or hiking sticks. it excels at holding one end of a hiking pole extended and hung horizontally to serve as a field-expedient clothes line.
I hope this helps someone.