Stealth speed cameras...myth or fact

[Grahamm]

I thought speed cameras had to  be visible these days, as in high vis?

DfT Guidelines:

Visibility of fixed cameras -

"Speed camera housings must be coloured yellow. Housings must be visible to road users and not obscured e.g. hidden behind bridges, signs, trees, bushes or other types of obstacles. The minimum visibility distance should be 60 meters where the speed limit is 40 mph or less and 100 meters for all other limits."