Methods and apparatus are disclosed for detecting a wirelessly detectable object. The wirelessly detectable object may include an antenna and a passive variable-frequency transponder circuit that is communicatively coupled to the antenna. The passive variable-frequency transponder circuit is powered by an interrogation signal received from an external source, and returns via the antenna a wireless response signal having a frequency that varies over a single interrogation cycle. Such variation may be based upon a decaying control voltage that controls the value(s) of one or more components in the passive variable-frequency transponder circuit and/or a switch that selectively couples and decouples components in the passive variable-frequency transponder circuit. The variable frequency may provide a characteristic signature for the wireless response signal that may be used to identify the wirelessly detectable object.