We get blips of 10V on the top string as car headlights briefly hit part of those panels as cars turn down our road.
If a car stops at the top of the road with headlights on for a few seconds then that can be enough to put 10V on that string for 20-30 minutes. The PV cell has nowhere to discharge to so the voltage takes some time to decay.
We have short strings (5 and 4) so those of you with larger arrays will see longer decay times - which may last all night depending on how much light the PV cell was exposed to.
This is only obvious on HA/GivTCP graphs as the sample period (5 minutes) on the portal means the data there is misleading in this case.