  Users of quantum computers must be able to confirm they are indeed
functioning as intended, even when the devices are remotely accessed. In
particular, if the Hilbert space dimension of the components are not as
advertised -- for instance if the qubits suffer leakage -- errors can ensue and
protocols may be rendered insecure. We refine the method of delayed vectors,
adapted from classical chaos theory to quantum systems, and apply it remotely
on the IBMQ platform -- a quantum computer composed of transmon qubits. The
method witnesses, in a model-independent fashion, dynamical signatures of
higher-dimensional processes. We present evidence, under mild assumptions, that
the IBMQ transmons suffer state leakage, with a $p$ value no larger than
$5{\times}10^{-4}$ under a single qubit operation. We also estimate the number
of shots necessary for revealing leakage in a two-qubit system.
