Rohypnol in particular should be easy to pick up, Flunitrazepam has a halflife of 18+ hours and at least one metabolite has a halflife of 36-200 hours. In fact according to this - http://www.ncjrs.gov/pdffiles1/nij/grants/201894.pdf - it can be detected in urine for 5 days and hair for a month.

If people are unlikely to report, or if drugs that are metabolised faster are used then sure, we may not have good data. But then all we can really say is we don't have good data.