I think Judy may have been describing a Meth Tweeker's "lab".
If the neighbor is growing on a fairly large scale, it will result in a high electric bill, and possibly a recognizable IR pattern. The overflights by a plane or chopper to detect the IR pattern, over multiple days are VERY expensive, so police don't do it for the Hell of it. And checking the utility bills for a house would require a search warrant, which would have to be based on probable cause.
Ignoring the legality issue, commercial marijuana production results in a hot, high humidity environment. In a normal residential house that's going to result in mold growth that, over time, will be of such a scale that the house will become a tear-down. So the DU OPer has a valid interest aside from pot being illegal in not having the neighboring house being used for commercial pot growth. Who wants to live next door to a monstrous mold infestation, and then the demolition of that house (which could spread mold all over the neighborhood!)?