So I was starting to familiarise myself with QKD when I came across this video on YouTube breaking down the process (I believe this is the E91 protocol but I could be mistaken) using an animation that was rather easy to follow. However I have one question regarding the section where the eavesdropper scenario was shown.
We see that Alice and Bob receive the following keys with the bold parts being the "intercepted" qubits
A: 1 1 1 0 0 0 1 1 0 1 0 0
B: 1 1 0 0 1 0 1 0 0 1 0 0
Now obviously, the difference in these keys is an indication of an eavesdropper. However, it is also evident that all not all the qubits show signs of interception. For example qubits 2,6,7,11,12 still match between the keys.
My question is this, is it possible that the eavesdropper could leave no trace of the interception (assuming he's lucky enough since the process are assumed to be completely random)? If so, what are the "rules of thumb" that one has to consider in order to maximise the likelihood that an eavesdropper leaves a trace on the keys (for example an optimal key length, detector sequence, etc)?