I’ve had a rash of finding hardwired detectors where the interconnect wire is incorrectly installed.
(And, a fair number of battery alarms found in a hardwired spot).
How do you verify that hardwired detectors connect to each other? When I press the button on such a detector, the beeping is so non-specific in location, I generally can’t make heads of tails of which detectors alarmed and which ones remained silent.
I’ve had the residents on occasion close a door and verify the connection, but this is not a universal solution.
A few systems, like the Kidde P4010DCS-W will give a detector count, but most do not.