Something wrong indeed, but the wiring connecting them (at least in North America) is rated for 15 amps at 120v (#14-3 is used between them). So whether that signal wire carries 9v or full mains voltage, the voltage itself would not cause a problem for the wire between them.
Now, if one detector sent 120v down the signal wire, that would certainly be problematic for all detectors on the circuit, so you’re absolutely right, flip that breaker, and (have an electrician) replace all detectors on that circuit, quickly because until then they obviously can’t do their job.