Since I started WSPRing 3 years 38 weeks ago on 30M, I have heard data signals above the Wspr slot. Of course you actually listen to the audio when starting Wspr.
Just this morning I heard a JT65 signal inside the Wspr passband, starting up WSJT I monitored the JT65.
It was a C91 station sending a CQ, also on the WSJT waterfall was my old data signals.
Looking at the screen capture it looks like a narrow FSK signal from one station to another, one strong (S9) and the other weaker signal in return, then stops.