go to http://jira.atlassian.com/secure/

log out if automatic login is active

return to the home page
use the quick search box on the home page
type in a 3-4 digit number (possible issue key) for example '567'

you get a single result :

http://jira.atlassian.com/browse/567
which is an error page
the URL is built without a project key

This doesn't occur as soon as you click on 'browse project' in the navigation bar (then, a default current project seems to be defined and used in building the URL)

Limitation of the quick search on issue keys not going accross projects ??