>>I've seen plenty of documentaries in which UK policepersons will sneak up on unsuspecting people in railway stations or similar places and take their bag (which they are not watching properly) and wait until the person notices it is missing (usually when then setting off) before advising them of the error of their ways.
It used to be common practice for police on foot patrol at night (OK, I know, I'm nostalgic) to rattle the doors/gates to commercial premises to check they were secure and if not, to pop in to check that all was well and if possible, later advise the owner to apply better security.
Both of the above are laudable, and done with the best of intent.
1)Both of those are done by POLICE not VIGILANTEs
2)It's not illegal for me to pick up your bag. I'm not going inside to rummage about and then handing it back. Entering your computer system uses cycles and you cannot guarantee his code is bug-free.