CUNY Academic Commons

This is a fun one for a rainy day

Tahir, this might be a good place to dive into BuddyPress's internals.

The avatars in question are shown in places like http://commons.gc.cuny.edu/activity/. Filter by "Show Blog Comment" and you'll see that some items have a dummy avatar. (There is a related bug, which is that they read "commented on the blog post" - missing a subject.)

For avatars, start by filtering 'bp_get_activity_avatar'. Here's a framework:

function cac_anonymous_activity_avatar( $html, $params ) {
    global $activities_template;

    // We're only concerned with avatar requests where the object ID is empty.
    if ( ! empty( $params['item_id'] ) ) {
        return $html;
    }

    // We'll only have enough information to do the lookup if we're in an activity loop. Here's a clunky but effective way to check whether this is the case.
    if ( empty( $activities_template->activity ) ) {
        return $htmls;
    }

    // Then, use the $activity values to fetch the source comment, and get enough information about the commenter to look for an avatar.
    // Then rebuild the $html using the new avatar URL
    // It'll probably look something like this:
    $activity = $activities_template->activity;

    $blog_id = $activity->item_id;
    $comment_id = $activity->secondary_item_id;

    switch_to_blog( $blog_id );
    $comment = get_comment( $comment_id );

    // $comment should have enough info for you to assemble a Gravatar URL. See especially $comment->comment_author_email

    restore_current_blog();

    // then do something to rebuild the $html markup that must be returned from this callback
}
add_filter( 'bp_core_fetch_avatar', 'cac_anonymous_activity_avatar', 10, 2 ); 

This is pretty rough and untested, but it may be enough for you to get a feel for the WP filter system and some of the fundamentals of BP/WP's internals.