code: public void sessionRemoved(final MemcachedBackupSession session) {
          if(!_sticky) {
              if(session.isLocked()) {
                  _lockingStrategy.releaseLock(session.getIdInternal());
                  session.releaseLock();
              }
              _invalidSessionsCache.put(session.getIdInternal(), Boolean.TRUE);
          }
      }
docs: Is invoked when a session was removed from the manager, e.g. because the
  session has been invalidated.
  
  Is used to release a lock if the non-stick session was locked
  
  It's also used to keep track of such sessions in non-sticky mode, so that
  lockingStrategy.onBackupWithoutLoadedSession is not invoked (see issue 116).
  
  @param session the removed session.