Weird, I've only ever had one swab test, the rest have been piss tests. Piss tests seem to have come on in leaps and bounds over the last decade and are now quick & reliable, though AFAIK they only show methadone, morphine, benzos, cocaine & amphetamines (I think)..
However, when I was sent for a drug test from work following a fortnights phenazepam binge, that was a swab test and it came back positive for barbiturates (amongst everything else).. this totally stumped me because I've never knowingly taken barbiturates in my life. This sez a lot about the quality of the heroin about at the time...