Some talk of burying the battery in one of the tubes - down or seat.
That would mean no external cables, but there would have to be a charging port somewhere.
Then there's still the weight and the activation button.
A torch shone down the seat tube would reveal the motor, as would even minor dismantling of the bottom bracket.
A UCI blazer would have to be spectacularly incompetent to examine a Gruber bike and miss it.
UCI bikes are artificially made heavy to meet the minimum weight limit, so a smaller less powerful version could easily be fitted. An activation button could easily be concealed too, nevermind being remotely operated. Charging ports don't need to be easily accessible as you have team mechanics fettling the bikes every night.
I agree that a strip down by the UCI would obviously bring all this to light. However, how many times have they actually done this.
Personally, I don't think it is happening, but it is very possible.that it has happened in the past. Cookson seems to know it has, but cannot prove anything.