PAXBOY
On the last three occasions I've travelled from the same South West England airport and returned from the same major Scottish airport the metal detector arch in England has not actuated but the Scottish one has.
Each time I was wearing exactly the same clothes and shoes outbound and inbound and took care to have nothing in my pockets.
Last week I mentioned this to the security officer at the Scottish airport. He was perfectly polite and friendly about it but could not or perhaps chose not to offer any explanation.
Like you I suspect the arches are set at different levels at various airports.