Sonar detection has failed , sir !

[CapitanPiluso]

Playing Barents Sea Loiter mission I get a radio message about a russian SAG (probably escorted by a sub) facing northwest which I must engage.
My TA gets a faint 50 hz line, and nothing comes from sphere and hull arrays in NB, even going to PD shows no ESM radiation neither visual contacts.
But after an hour or so I get a TIW ... and really no idea where this torp is coming from, my TA keeps on showing a single line.
But to my surprise, I turn "truth on" to find that my TA was always trackin the last ship in the formation, and never could get the enemy sub ( who has detected me from 26 nm)

Some tactical idea will be great.THX