Please be aware that issues like this can happen when using new and old detectors.

Detectors do not ship unique IDs and shows battery low messages (probably the old type).

Detector must be deleted – not only from the app but also from the group.
Something similar can happen if a detector is not enrolled anymore (simply deleted from the app, but still alive as part of the same Cavius group) – this will be detected when you perform a test because an unknown sensor starts beeping.