  Decoherence of superconducting transmon qubits is purported to be consistent
with surface loss from two-level systems on the substrate surface. Here, we
present a study of surface loss in transmon devices, explicitly designed to
have varying sensitivities to different surface loss contributors. Our
experiments also encompass two particular different sapphire substrates, which
reveal the onset of a yet unknown additional loss mechanism outside of surface
loss for one of the substrates. Tests across different wafers and devices
demonstrate substantial variation, and we emphasize the importance of testing
large numbers of devices for disentangling different sources of decoherence.
