According to blind test with various trained dogs, the opposite of what you stated is true. From the study -
" 68 Labrador retrievers, 61 German shepherds, 25 Terriers and 10 English Cocker Spaniels, of both sexes in each breed, were used. Altogether 1219 experimental searching tests were conducted. On average, hidden drug samples were indicated by dogs after 64s searching time, with 87.7% indications being correct and 5.3% being false. In 7.0% of trials dogs failed to find the drug sample within 10min. The ranking of drugs from the easiest to the most difficult to detect was: marijuana, hashish, amphetamine, cocaine, heroin. German shepherds were superior to other breeds in giving correct indications while Terriers showed relatively poor detection performance. Dogs were equally efficient at searching in well-known vs. unknown rooms with strange (i.e., non-target novelty) odors (83.2% correct indications), but they were less accurate when searching outside or inside cars (63.5% and 57.9% correct indications respectively). During police examination trials the dogs made more false alerts, fewer correct indications and searching time was longer compared to the final stage of the training. The drug odor may persist at a site for at least 48h. Our experiments do not confirm the recent reports, based on drug users' opinions, of low drug detection efficiency."
And when it went to the Supreme Court, they ruled a unanimous decision in favor of the dog. "The United States Supreme Court returned a unanimous decision on February 19, 2013, ruling against Harris and overturning the ruling of the Florida Supreme Court.
[29] In the unanimous opinion, Justice Elena Kagan stated that the dog's certification and continued training are adequate indication of his reliability, and thus is sufficient to presume the dog's alert provides probable cause to search, using the "totality-of-the-circumstances" test per
Illinois v. Gates. She wrote that the Florida Supreme Court instead established "a strict evidentiary checklist", where "an alert cannot establish probable cause ... unless the State introduces comprehensive documentation of the dog's prior 'hits' and 'misses' in the field ... No matter how much other proof the State offers of the dog's reliability, the absent field performance records will preclude a finding of probable cause."