Crime
This skill covers all criminal activities.
It covers both the use of and theoretical knowledge. So a detective needs a crime skill too, so he can figure out what has been done how.
Also if you want to build security systems, you need to know how a criminal would go about business to create useful countermeasures.
Some examples of specialisations:
Disarm Traps
Disguise
Escaping bonds
Forgery
Hide
Investigation
sneaking around
picking locks
picking pockets
Lip reading
Shadowing
Sleight of hand
Computer hacking
Building and use of simple explosives
Securty systems