  Understanding and suppressing sources of decoherence is a leading challenge
in building practical quantum computers. In superconducting qubits, low
frequency charge noise is a well-known decoherence mechanism that is
effectively suppressed in the transmon qubit. Devices with multiple
charge-sensitive modes can exhibit more complex behaviours, which can be
exploited to study charge fluctuations in superconducting qubits. Here we
characterise charge-sensitivity in a superconducting qubit with two
transmon-like modes, each of which is sensitive to multiple charge-parity
configurations and charge-offset biases. Using Ramsey interferometry, we
observe sensitivity to four charge-parity configurations and track two
independent charge-offset drifts over hour timescales. We provide a predictive
theory for charge sensitivity in such multi-mode qubits which agrees with our
results. Finally, we demonstrate the utility of a multi-mode qubit as a charge
detector by spatially tracking local-charge drift.
