Anyway back to the matter itself.
Your padlock theory.
There are only a very limited range and size of padlocks that can fit such a holdall. http://farm7.staticflickr.com/6112/6302 ... db24_z.jpg
This could be a good one over and above the traditional padlock, as it clips into place rather than needing the alignment of the swiveling thing. However, this lock takes vision to unlock, unless you can keep track with your finger. However, the whole actually locking the case just seems silly. If you were attempting this you would just fix the lock to the zip without clipping it shut.
However, I would consider it highly unlikely that this development was so effective that it meant a first time success, plus we have no evidence that the padlock changed on the ill fated attempt.
Also, it needs to be considered that the fingers become the eyes when attempting these things, giving yet more reason for any padlock to have DNA.
Another observation I like to put forward would be how much DNA was found in the holdall? Clearly we don't know this but it would assist the investigation greatly. Naked skin against a coarse surface, even during one attempt would cause a large amount of skins debris. And if this was a regular occurrence you could scrap together a huge amount of skin as opposed to hardly any if he entered the bag postmortem.