Without popping the case for the ERS I couldn't tell you what sort of signal it could produce. It deals with a pulsating voltage so if it has reactive components such as chokes and capacitors, theoretically (if the right fault conditions exist ) it could produce a voltage spike.

Also a faulty solenoid coil can produce a spike as the magnetic field collapses. A lot of solenoid coils can have a zener diode or capacitor to kill the spike.

I would do the test again with another meter and use a knockometer (plastic handle of a screwdriver) to gently tap test the components and wiring harness.

Did the solenoid click in and out with a 12 volt feed?