||Assuming for the purposes of this answer that, as you say, you do not yourself ingest cannabis, your theory of passive absorption from being in an environment that shares air ducting with rooms where cannabis is being smoked would be theoretically possible. The same argument holds and is accepted for second-hand tobacco smoke.|
Given the values of 15.2 and 15.6 ng/ml that were reported on your two positive (failing) GC/MS results, where the cut-off limit for a positive test result is 15 ng/ml, it is possible the one test that was negative was just below 15 ng/ml, which would pass. It also depends on what threshold the individual company declares as a "positive" value. While there are government standards for what the pass/fail cut-off levels should be, individual companies are not bound by those levels. While unlikely, it's possible that a company could choose to declare any level above zero as a failure, or alternately, to select a higher cut-off level than 15 ng/ml.
Given the circumstances -- three specimens, all at the same lab under similar conditions on the same day, with different results -- challenging the results could be an option. But because the failed tests were both so close to the cut-off level, and some variation in results would be expected, passing two out of three isn't THAT weird.
If you are truly exposed to second-hand smoke from cannabis, submitting to hair testing could yield similar positive results. The low levels that you report do not suggest recent use or chronic use. It would be interesting to see an analysis done of air sampled from the ducting in your building.
Unless you move somewhere uncontaminated by second-hand cannabis smoke, you continue to run the risk of testing positive in the future.
Job Hunter. Detection Cut-Off Too Low. Erowid.org.