Then look for the seam or see if there's some actual wooden skirting under it. He'd probably drill into dado trunking thinking the sockets were just mounted on it. Common sense goes a long way.
This is getting a little silly, but the fact is that he knew that there was wooden skirting below it but thought that, as with other things in his newly-acquired house, bits of ghastly plastic 'prettification' had been added on to existing wood 'to give a more modern look'.
As BAS has said, the problem/risk (which is very small, either way) results from hiding wiring - whether behind plastic or wood. ... and, of course, those who undertake DIY on skirting cannot be guaranteed to possess common sense!
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