black cut out fuses

Is that a non-contact sensor? I.e. would you expect it to light stroking the jacket of a dry PVC single insulated wire with a copper core that was live ? If so there is probably nothing to worry about - there is always some electric  field around live parts and a cut out is a place where the spacing between the live parts and the associated neutral/earth is wider than normal, so the volume where that field will be detectable is larger. However, if the probe is one that normally only lights when you actually connect its tip to a live part, then assuming its not dripping wet, you may be looking at plastic that is starting to lose its insulating properties. Even so degrading insulation not really dangerous until a thumbprint size contact picks up a good fraction of a mA. (*)
The fact the other sensor shows no response suggests if there is any unusual  leakage developing at all, it's not yet at a significant level.
Mike.

* and at that point, avoid touching it with your thumb or finger tips while other bits of you are solidly earthed - you may only feel a tingle, or maybe worse ... ;-)

Is that a non-contact sensor? I.e. would you expect it to light stroking the jacket of a dry PVC single insulated wire with a copper core that was live ? If so there is probably nothing to worry about - there is always some electric  field around live parts and a cut out is a place where the spacing between the live parts and the associated neutral/earth is wider than normal, so the volume where that field will be detectable is larger. However, if the probe is one that normally only lights when you actually connect its tip to a live part, then assuming its not dripping wet, you may be looking at plastic that is starting to lose its insulating properties. Even so degrading insulation not really dangerous until a thumbprint size contact picks up a good fraction of a mA. (*)
The fact the other sensor shows no response suggests if there is any unusual  leakage developing at all, it's not yet at a significant level.
Mike.

* and at that point, avoid touching it with your thumb or finger tips while other bits of you are solidly earthed - you may only feel a tingle, or maybe worse ... ;-)