Kill 15 Lusha at the Lusha Dig to see if you can find any trace of the Siren's Call.
It can't be coincidence - the flute must be in the area, and it must be in the possession of these Lusha! They won't hand it over easily; they killed the entire HUD team that found the flute in the first place. You'll have to take it by force, even if it means killing them one by one.
They MUST have it!
You've certainly tackled your fair share of them, and no flute? It can't be…