Tamper-respondent assemblies and methods of fabrication are provided which include at least one tamper-respondent sensor and a detector. The at least one tamper-respondent sensor includes conductive lines which form, at least in part, at least one tamper-detect network of the tamper-respondent sensor(s). The detector monitors the tamper-respondent sensor(s) by applying an electrical signal to the conductive lines of the at least one tamper-respondent sensor to monitor over time for a non-linear conductivity change indicative of a tamper event at the tamper-respondent sensor(s). For instance, the detector may monitor a second harmonic of the electrical signal applied to the conductive lines for the non-linear conductivity change indicative of the tamper event, such as an attempted shunt of one or more conductive lines of the tamper-respondent sensor(s).