What is indicated when a K-9 alerts to currency?

When a K-9 alerts to currency, it indicates that the dog has been trained to recognize specific scents associated with substances often found on tainted currency. Currency can carry residues from drugs or other illegal materials, and these odors are what a K-9 is specifically trained to detect.

The correct choice suggests that it's not just any currency that would trigger the alert; it is particularly currency that has been contaminated with certain substances, such as drugs. This shows that the K-9 is discerning not merely between clean and dirty currency but is specifically responding to cues that indicate that the currency is associated with illicit activities, rather than other unrelated smells.

To emphasize further, the alert does not depend on the currency being visibly contaminated, nor does it require residue from any currency to prompt an alert. Additionally, while some K-9s are trained to detect counterfeit currency, the primary purpose related to alerts in this context is typically geared towards identifying tainted or drug-related substances.