What condition would most readily signal the presence of a meth lab?

The presence of an odor similar to ammonia or cat urine is a significant indicator of a meth lab. These distinct smells arise from the chemical processes involved in the production of methamphetamine. The odors can stem from various toxic substances used in the manufacturing process, including ammonia itself, acetone, and other solvents. Noticing such smells can be a strong warning sign that illegal drug activity is taking place, as they are characteristic of meth production.

The other options may point to suspicious activities or conditions but do not specifically indicate the presence of a meth lab. Visible structural damage might suggest neglect or other issues but is too broad to specifically denote drug production. Excessive security measures could imply that an individual is attempting to shield something from view, but this is also not unique to meth labs. Frequent visitors at odd hours can indicate unusual activity, but similar patterns can arise in legitimate business operations or social gatherings. Thus, the odor is the most direct and recognizable sign of a meth lab's existence.