Doing surveillance work or using an unseen marker to aid tracking that attaches with ceramic magnet might have this in their tool box . This could be adjusted to visible light range.
This is IC ILC7555 which is CMOS version of 555 which has speeds of 1MHz. Normal 555 is speeds of 500 kHz. Six Infrared diodes are also used.
To see marker use snooper scope (night vision app for your smartphone) or CCD ( charge coupled device ) camera.
