"Surely they must have some kind of echo based technology that could detect objects awash ?"
It's a new fangled thing called "Sonar". But don't tell anyone. It's top secret!
Why would a submarine care about relatively small pieces of <something> floating on or close to the surface?
I think some here have been watching too many WWII sub movies or something. The use of active sonar (i.e. the issuance of a sonar "ping") is a bit like suddenly standing up in the middle of a battlefield in a fire-glo orange suit and yelling "I'M HERE!", which is pretty much the last thing a modern submarine wants to do.
Vastly more effort has gone into passive listening sonar, which is certainly not going to help.
The French sub that tried to find AF447 had to have a special software update from Thales to allow it to listen to the 37.5kHz pinger frequency, so any sub that happened to be within cruising distance of the suspected crash zone is unlikely to be of any use at all...