Discussion on answer by Jpe61: Why would a fighter drop fuel into a drone?

@U_flow I think it indeed fuel coming from the afterburner, as I have guessed, but did not know for sure. See, eg.g., metabunk.org/threads/…

According to various sources the SU-27 has a single fuel jettison outlet in the tail boom (between the two engine exhausts). But like Vladimir mentioned it seems to be possible to also use the afterburner fuel lines to release fuel (link)