Why save_post_$(custom_post_type) is fired even if I am not already saving a post?

When you choose “Your CPT > Add New”, WP calls get_default_post_to_edit(), which actually creates an “empty” post (with 'post_status' => 'auto-draft') and then calls wp_insert_post(). This is what is causing your save_datasheet_meta() function to be called before you think it should.

Hence, generally you should add some additional sanity checks to the beginning of any func you hook into save_post, ala:

function
save_datasheet_meta ($post_id, $post)
{
    if ('auto-draft' == $post->post_status) {
        // bail, because your nonce will not be set at this point
        return ;
        }

    // post{,-new}.php enqueue some JS which periodically saves the post_content,
    // but NOT any post_meta
    // so, a `save_post` hook func that only cares about updating post_meta should
    // bail when DOING_AUTOSAVE is true
    if (defined ('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
        return ;
        }

    $nonce = wp_verify_nonce ( plugin_basename(__FILE__), 'datasheet_meta_nonce');

    if (!$nonce) {
        die ("Security check failed");
        }

    // save post_meta's here

    return ;
}