Could Malaysia Airlines Flight 370 have sunk without leaving any floating wreckage?
Yes, but only under highly unlikely circumstances.  An airplane contains all sorts of materials that float, from fuel to seat cushions to the plastic cups in the galley.  In order to sink without leaving any floating wreckage, all of that material would need to be trapped within the airplane while it fills with water.

In order for this to happen, the airplane needs to land intact on the water.  As any number of ditchings indicate, this isn't going to happen -- even US Airways 1549 had the rear of the fuselage tear open on landing.  More often, the engines or wings will tear off, leaving oil and fuel slicks.