5by5,
The scenario you suggest is plausible.
As the body metabolises alcohol quickly, it is possible to be clear
(or just below the breathe test limit) but for a urine test to be positive.
Whilst it is unlikely that a policeman would require a further sample down the station - it might happen if, say, you were particularly bolshy or he didn't like your handlebars or the cut of your jib (or maybe because he did............).
A test at the station could also be more comprehensive and could be extended to include "drugs"....... (say by sending the sample to a laboratory for further testing).