Would it be possible for someone to have consumed a soporific drug, such as Rohypnol, and remain unaware of the fact after recovering?

Technically the answer to this question is yes. Especially if a person is unfamiliar with the feeling and effects of this type of drug, it's possible (some would even say likely) that they may simply pass out and wake up later a little confused as to why they were so exhausted.

Also, many of these types of drugs also cause significant memory loss from the time between when the onset of the drug occurs and when the person actually falls asleep. Even if they never actually pass out there may be a period where they act disoriented or confused and later have absolutely no memory of their behavior or anything that happened. (This would vary from approximately 30 minutes to several hours depending on the particular drug consumed, the person's tolerance, weight, and many other factors.) While some of the drugs might leave you with a hangover there are others where a person wakes up feeling rested and refreshed.

As a side note, they may also cause a person to lose some of their inhibitions. This is why drugs like Rohypnol and GHB are often used in sexual assaults. The offender doesn't think that a person might want to have sex with them while they were sober so they either offer them a drug like this in hopes that it will loosen the person up and increase their chances or they slip it to the person secretly with the specific intent of having sex with them when they are incapacitated. If the person takes the drug knowingly then the offender can easily convince others that the victim did so and was also an eager and willing partner. Since the victim's memory will be extremely fuzzy or completely blank there is less of a chance that they will remember enough details to feel comfortable reporting it.

Here are some related links that I added at the request of a user:

Rohypnol Information: http://www.whitehousedrugpolicy.gov/publications/factsht/rohypnol/
http://www.urban75.com/Drugs/rohypnol.html
http://www.erowid.org/pharms/flunitrazepam/flunitrazepam.shtml

GHB Information:
http://www.erowid.org/chemicals/ghb/

Information on How to Prevent Drink Spiking and Date Rape:
http://springbreak.studentads.com/Safety/Alcohol-What-Parents-Need-to-Know