The speed indication on my ebike is acting up - most of the time it's showing a correct reading, but it occasionally drops to impossibly low values like 2 km/h for a few seconds while riding, then it recovers. Sometimes the Bosch controller will also briefly indicate "Error 503", which is an issue with the speed sensor.
Searching online for this issue will bring up many videos like this one, where it's explained there is a magnet on the rear wheel and a sensor on the chainstay, much like an ordinary bike computer in traditional bikes, and that this magnet sometimes moves out of position, causing the error.
The trouble is that on my bike, I can't find anything like that - there's no magnet on the wheel and there's nothing like that sensor on the chainstay:
In that second picture there are two cables running into the frame - one is the brake cable and the other goes somewhere between frame dropout and the brake disc, I can't see where it ends without taking the wheel off. Here's what it looks like from the bottom (taken at the point where it exits the chainstay, looking back):
Does this bike use some kind of different speed sensor setup?