Locating Missing Airplanes using Remote-Sensing Satellites
Civilian earth observing SAR satellites do not always operate over empty swaths of oceans (specially in the middle of Indian Ocean) in order to save power. They are not designed to track a fast moving target and have repeat cycle of at least a few days. Then there is the issue of resolution. High res acquisitions modes (with satellites like TerraSAR-X) are pre-programmed in advance.

So in theory it is possible that a flying aircraft might show up in the amplitude image (a pixel or two wide) if a satellite was indeed flying over the Indian ocean and acquiring data over the area where the plane was flying. But then all you have is a single image. And once the plane is under water you wont see anything.