I would presume he wouldnt be seen when flying that fast
as light is two orders of magnitude faster than sound (30km/s vs 0.3km/s) I presume that anything travelling at a speed so fast it cant be seen would also be invisible to radar...
this would depend on the sampling rates of the detector though...
ie we dont see supes at those speeds due to the speed of information processing in our retinas and our brains, not because he is actually travelling faster than light...
in L&C he cannot travel at supersnic speeds with a passenger... but he can "move his molecules" so they are permiable to light (3.06 Don't step on Supes cape)
I dont know the intricasies of radar which would make a difference here, but I presume you could make him invisible to radar solong as he didnot have a passenger...
