Most airlines allow some discretion if there is no physical sign of fire and there is reasonable cause to believe the indication is spurious, eg after cargo spraying. Or perhaps no hotspots on the thermal imaging and freezing conditions outside make an immediately available air bridge the least hazardous option.
Incidentally, the whole 'precautionary evacuation' via air stairs idea is only practical if the airport has plenty of advance notice. It can take 20 minutes at large airports (even in an emergency) for them to arrive.