Or an undisclosed radar/laser capability. Plasma can reflect radar and it emits light - a focused phased array could create local plasma somewhere that other craft pick up.

Ofc, once you#ve done that you may not want third parties to know. Something as banal as 'we made air glow a few miles away' can expose capabilities of some vital systems that you don't want third parties to know. Edit: I mean, with that information, someone could do some napkin math on what can still be seen and what can't (by e.g. a naval radar suite, focusing on a spot in the air using a phased array).