Infred does temp. Any bit of aircraft that has been in the sea for more than a few hrs will be the same temp as the sea.
I am not an expert in satellites, I know a little about it through a previous job, but not much. I am fairly sure we have, as yet, to develop an optical method of measuring density. There is XRF to do elemental analysis, but I don’t think satellites send Xrays (yet). (I could be wrong).
There is satellite based radar for interferometry purposes, used fro topology mapping. I do not think it could detect half submerged moving around stuff though.
There might be other methods, but we would have heard about it, the reporters are desperate to find a new way of explaining this. The images released are optical,and it is inferred that the “debris” has been spotted optically.
What I am not sure about is I think that the observation satellites are making passes, so they only take one image, and then on to the next sector. So they cannot discriminate if an object appears/disappears (ie the white tops on a wave). If they could take lots of images of one spot I think we would have seen them too. But I am not 100% sure about this..