Well, it's actually not picking up the marijuana. The "beam" which is actually a infrared recepticle, picks up on the heat generated of the light bulbs used in growing the green. And the police also can look at your electric bill, to see if the energy used in prior months is greatly lower than that of recent months.

If you have like 1-4 plants, probably not a big deal. More like a huge operation like 50+ plants. Possibly less than that.

Dave