Are there any disturbances caused by wire of socket which not being used?
If upstream of the wiring that leads to the AC power socket there is equipment that adds noise to that upstream feed line then, that noise (as a voltage fluctuation) will also be seen down stream at the unused AC power socket. That can capacitively couple to the CAT-5 cable (or any cable within reason) and it could cause cross-interference problems.

Having said that, there are many circumstances where it might not cause cross-interference problems. No theory behind that just plain bad luck on how the two cables (AC power and CAT-5) might be physically disposed with respect to each other.