A system and method for detecting concealed cargo, such as explosives contained in a closed vehicle trunk, by characterizing the transfer function between sets of wheels of the vehicle and automatically detecting changes of transfer functions due to changes in mass loading. The system and method contacts a first tire of the vehicle and transduces a signal such as a sweeping acoustic signal into the vehicle, simultaneously collecting the signal transmitted and attenuated through the vehicle to an second tire. The collected signal is analyzed and compared to previous transfer function characterization results to detect significant changes in the transfer function. If a significant change is detected, a system operator is alerted to prompt further preemptive action, such as a full search of the vehicle.