Saw it too - about 4000' on FR24 if I recall. Possibly using LIDAR for the 3D mapping, though I've seen one trundling up and down London at low level a few times at perhaps 2000' and also rather higher on occasion, some of which I assumed was LIDAR mapping.
There was what sounded like a P-68 (sounds the same as a Duchess/Seminole/Cougar) orbiting SE London at about 9000' last Thursday morning. Couldn't see it on any of the radar apps and didn't have any bins on me for a better i/d other than it appeared to be high wing. Certainly not one of the regular "listeners" which I think all have turbines.