So is there a chance that if we do see a mirror electron once in a while we may mistake it as normal electron.

This is part of the background, indeed. I recall that the background expectation is the observed distribution (there is no signal).

In the case of a signal, we should observe a global difference in the distribution of the datapoint. We should get the background (the data points, I recall) plus what we can get from the solid contours. In other words, we have enough discriminating power.

I think that now I answer the question (otherwise, please come back again to me :D )