wrote:

I take it this voltage presence is wrong? Anyone any pointer as to
what could be causing this?

A digital multimeter I take it?

If so the chances are you are seeing something called capacitive coupling.

Cable runs will have a small but detectable capacitance between the
conductors. This allows a tiny amount of current to flow between the
conductors even though when they are not connected to each other (and in
fact separated by insulation!). This allows some of the disconnected
sections of cable (i.e. switched off bits) to "charge up" toward some
fraction of mains voltage. A digital multimeter will tend to show this
since it places very little "load" on the cables when carrying out the
measurement. (Conventional meters place a higher load on the circuit
under test and this tends to discharge the cables).

You may well find the effect is more noticeable on two way switched
circuits since they will have longer runs of cable.

The flashing you can see with the CFL bulb is quite common, and seems to
be an extension of the same problem. The small leakage current due to
the capacitive coupling is also enough to charge capacitors in the input
circuitry of the lamp. Once these has accumulated a sufficient level of
charge, it is enough to momentarily "fire" the lamp. This discharges the
circuit, the lamp goes out and the process starts over.

--
Cheers,

John.

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