After the stage the doping controls check the regular bike, but are unaware of the existence of the doped bike.
Cancellara had a mechanical issue and did a first bike chance. The main mechanic jumps out of the car and gives him a new bike. A few km's later Breschel has a real issue and requests a new bike, but the main mechanic isnt in the car anymore resulting in Breschel getting the wrong bike, losing a lot of time and never returning to the front. A few km further the images show Cancellara suddenly sitting on his original bike.
What happened, at the point where cancellara did his first bike change, the stage makes a loop. The mechanic had to walk 500 meter to get on the other part of the stage. What does amateur footage reveal. In a turn everybody where makes the short turn, Cancellara goes wide where the main mechanic is waiting with the original bike and you see Cancellara do a lightning bike swap.
Its a sad thing but after all these years I am convinced Cancellara cheated.
(Stolen from a comment on https://www.youtube.com/watch?v=z6z7uUe0tVA)
You vastly overestimates that tiny battery and motor vs. the athlete's legs.
The only time it make sense is in sprints where it can contribute a significant torque increase for a short while. Over 100 km you might as well be better off without that weight.
There's a reason why all electric vehicles are heavy and burly looking electric bikes only have range of 20mi, and those batteries are not constrained by weight or shape to fit in a road bike tube.
The only time someone has been caught for motor doping was for a spare bike that they never used in that race.