AUK's current stance is that routes (routesheets) are advisory, not compulsory. So secret controls make no sense.
However some Organisers have the stance that their route (routesheet) is compulsory. Generally this can only be 'enforced' (yeah right) by the use of secret controls (real or virtual).
So a compromise position has been reached, as already described by Mal. Any leg of a route can be made compulsory by the addition of a secret control, somewhere in that leg, and this can be declared on the brevet card if that is what the Organiser wants. Very few events do this but it could in theory do away with multiple infos (spit).
The (very old) system of "there will be a secret control somewhere on this event so follow my routesheet or else" is just crap, sloppy, lazy organising, and good riddance.