"Gems will be installed to your default system location for gems. If your system gems are stored in a root-owned location (such as in Mac OSX), bundle will ask for your root password to install them there."

http://gembundler.com/v1.3/bundle_install.html

Thus without using sudo it will ask for your sudo password if it's installing system wide, which it does by default. If it doesn't install any new gems as they have all been installed previously it won't ask for your password.