The usekeys option determines the behaviour
of using keys.

Values are:

inventory: Only keys in the top level
inventory are used (default)

keyrings: Only keys in active keyrings
are used.

containers: Only keys in active containers
 are used.

Note that keyrings are just a specialized
container, so the containers will also 
find keys in keyrings.

Only active containers are used - keys will
not jump out of closed containers, but hints
will be given if you have keys in such containers.