In that case, the only thing that will work will work on units not in fires. Heat rises and for the most part, will curl right off if any heat gets somewhat nearby.
This might be an after thought, but they should've probably been screen printed directly onto the units before assembly.
Again, they should just monitor which homes their units go into and keep a log. Or make a big sticker for sticking on the front window stating.... 'This Home Protected by ABC Fire Detectors'.