Target/room designation system for night time CQB. (Air or spring powered.)
LEOs/Military Personnel would be using NVGs.
The system could be used to mark targets or cleared rooms, by spraying low-luminescent or IR-luminescent liquid (if using hybrid NVGs), or shooting a paintball with the same liquid. (Think 'glow stick' or IR 'glow stick'.)
Whether it would be useful in reality would have to be determined by the design and the engineer. But, it's an idea, nonetheless.
I guess it isn't far off from pepper spray, either...
"Such is the strange way that man works -- first he virtually destroys a species and then does everything in his power to restore it."