Many smoke detectors are polarity sensitive; if the wires aren't landed so the positive wire is on the "+" terminal and the negative wire is on the "-" terminal, the smoke detector won't work. Some smoke detectors are not polarity sensitive; if the positive and negative wires aren't landed correctly, the smoke detector will still operate.
Of course, if a wire is connected to a third terminal on the detector for an annunciator or relay connection, or if the smoke detector is used directly on a Signaling Line Circuit (SLC), the smoke detector is still polarity sensitive.
Long term, if the service technicians have mixed and matched different detectors in the system, some of the detectors wired correctly and some wired reverse, future service on the system can be a nightmare.
Just as a matter of principle, I read the installation instructions that come with the detector, and I always connect the smoke detector as if all smoke detectors are polarity sensitive. That way I don't have to remember which make and model is polarity sensitive.