You would have to be standing in the middle of a group of smokers that smoked nonstop for hours for it to show in your system. The concentration from 2nd hand smoke is nowhere near what you'd need to test positive. Relax, these guys are just playing with you.

From DrugTestingKits.com:

Is it possible to test positive for THC (Marijuana / Cannabis) from exposure to second hand smoke?

It is not! Urine concentrations of THC above the cut-off sensitivity level of the test, or a positive result, is possible only through use / inhalation. However, second hand smoke will be in the system, but not at the concentrations that would indicate drug abuse.

From Marijuana.com:

Can Secondhand Smoke Cause a Positive Drug Test?

In rare occurrences, secondhand marijuana smoke can cause you to fail your drug test. It is possible that secondhand marijuana smoke will raise someone to the 50 ng/mL level. However, extreme secondhand exposure is required. For instance, being in a closed car full of pot smokers for several hours might cause someone to test positive in a drug test the very next day. Non-smokers are safe in a ventilated area such as an average living room or garage where partygoers are smoking pot.