  We revisit a scenario of continuous quantum error detection proposed by Ahn,
Doherty and Landahl [Phys. Rev. A 65, 042301 (2002)] and construct optimal
filters for tracking accumulative errors. These filters turn out to be of a
canonical form from hybrid control theory; we numerically assess their
performance for the bit-flip and five-qubit codes. We show that a tight upper
bound on the stochastic decay of encoded fidelity can be computed from the
measurement records. Our results provide an informative case study in
decoherence suppression with finite-strength measurement.
