\version "2.18.2"

\score {
  \new Staff {
    \new Voice \with { \consists "Balloon_engraver" }

    % The example bellow is working
    \balloonGrobText #'NoteHead #'(3 . 4) \markup { "Note head 1" }
    g'2.

    % This is only working in chord <> construct
    <b'-\balloonText #'(3 . -2) \markup { "Note head 2" }>2.

    % e.g. this does not work
    %a'2.-\balloonText #'(3 . -2) \markup { "Note head 3" }

  }
}