Radar and reflections
With the right frequency radar, you would get a good reflection from the angular metal structure of a lander.
Unfortunately, the Marsis radar has a wavelength ~ 100m, so a 1m lander (or the debris from one) isn't going to be a good reflector.
Otherwise, we might be able to locate quite a few missing objects on Mars.
(I meant the large number of missing landers, not anything more unusual).
Maybe when MRO arrives in 2006, with its SHARAD radar using wavelengths down to 12m, we will start seeing distinctive bright reflections from the landers and crashers?