BuddyPress.org

It is confusing.

Something, probably a plugin, is most likely do a $current_user check
for example: is_user_logged_in()
in a file loaded via a loader.php with something like:
add_action( 'bp_loaded', 'some_function_that_loads_a_file' );

The codex recommends using bp_loaded, but in 1.7 the notice was added.
I’ve been using this instead:
add_action( 'bp_init', 'some_function_that_loads_a_file', 9 );

The notice only appears if you have, in wp-config.php, debug set to true.
define('WP_DEBUG', true);

Other than the notice, are you seeing anything that isn’t working probably?
You could ignore it, at your peril, and set debug to false.
Or you could deactivate your plugins one by one to see which one(s) is generating the notice.

It would be helpful to get a definitive explanation from one of the core devs.