We don't know that to be true.....
link
"Investigators affiliated with Johns Hopkins University in Baltimore and the Substance Abuse Mental Health Services Administration in Rockville, Maryland assessed the likelihood of second-hand smoke exposure triggering a positive urine test under varying environmental conditions.
Six non-users were seated with experienced cannabis smokers in an alternating manner in each of the three study conditions. 
Non-smoking participants were exposed to second-hand cannabis smoke for a period of one hour.
In session 1, marijuana smokers consumed cannabis of roughly 5 percent THC in a non-ventilated room. In session 2, smokers consumed cannabis consisting of roughly 11 percent THC under similar circumstances. Smokers in session 3 also consumed higher potency cannabis, but did so in a ventilated environment.
Short-term exposure to second-hand smoke of higher potency cannabis in a non-ventilated setting was significantly more likely to produce elevated carboxy-THC levels in non-users compared to exposure in the other conditions. (Carboxy-THC is the primary inert metabolite of THC.) In one subject, 
exposure in this setting produced carboxy-THC levels above 50ng/ml, the standard cut-off for a presumptive urine screen. Several other session 2 participants 
tested positive at levels below 50ng/ml but above 20ng/ml, the standard cut-off for a confirmatory 
drug screen.
Session 2 participants 
continued to test positive for the presence of carboxy-THC at levels of 10ng/ml some 30 hours post exposure."
So second hand smoke caused positive results for these non-smokers (up to 30 hours later!) in a medical study done by "investigators affiliated with Johns Hopkins", why would we see different results with Gordon?
The results of that test are actually pretty interesting, 
found here.  One of the non-smoking subjects had a reading of 6.9 ng/ml at one point, and then registered a 27.3 ng/ml an hour later (again, without smoking anything directly).  One to three hours later, again without smoking anything, they registered a 57.5.