Having said that, the glow could be coming from an engine. Perhaps it was Russian?
Pulse Detonation engine maybe?
Well, not neccessarily.
A key aspect of keeping secrets is compartmentalization of information.
The people that ordered the jets to scramble may have not known about any secret experimental testing of advanced aircraft.
I don't claim to know what it really was.
I'm just saying that one hand doesn't always know what the other is doing.