How about "A ticket check is about to commence, please have your
{tickets and passes ready for inspection}"
Nothing whatever happened.

I've heard this several times on central London bendies; there's always a
few that get off at the next stop, and a few that 'remember' to touch in
their PAYG Oysters.

But not as many as the Evening Standard would have you believe ;o)

And I've yet to see the announcement followed by an inspection.
Real ones have no warning.

(The boy who cried wolf will set in eventually, I'm sure)
--

Andrew