Geraint Jones was kind enough to send me a plot showing the actual data from the MIMI instrument that indicates the presence of a disk, possibly including ring arcs, around Rhea. He's also included some data from a similar flyby of Tethys, in which there's no evidence for such a ring. (Tethys is actually smaller than Rhea; the diagrams are scaled so that the two moons look the same size, for comparison purposes.) You can see that, for Tethys, the squiggle representing the flux of electrons, is at a pretty constant level of about 500 counts per second until Cassini passes into Tethys' shadow, where it drops to below 30 (the scale is logarithmic). The squiggle looks very different for Rhea; as Cassini approaches Rhea there's a steady dropoff from 2000 to about 1500 until it reaches Rhea's shadow, and superimposed on that are three symmetrical narrow dips, which is what Jones and his coauthors are identifying as possible rings or ring arcs.