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@Airhead
You could switch the power off (and lock it off using a special clamp), switch off all the breakers, open up the CU and take a wander lead (any wire long enough to reach the CU from your mystery wire) connect it to the live (red or brown) of your mystery wire and wander back to the cu, use the continuity function of a multimeter connected to the end and then find out if it's connected to any breakers by probing the live terminal on each breaker to see if you have continuity.
Alternatively there are wire tracing units that work on dead circuits (switch the whole house off first) available for less than £20. Generally called fuse finders or in the automotive trade, wire tracers.
If you need to leave a maintenance free junction on it then look for WAGO connectors and boxes, any maintenance free joint should not have screw terminals as they can and do loosen due to several factors (heat/cold, cold flow of copper & AC pulsing being a few reasons).
So, yes an electrician could trace it.