What is life? How can we identify it?
Those are tough questions. Every time one scientist thinks he's discovered signs of life on, say, Mars, another scientist offers an alternate explanation for the data. The pattern began with the Viking missions in the 1970s and continues to, most recently, the localized methane eruptions recently spotted by both terrestrial telescopes and a current Mars satellite mission.
Fictional life-sign detectors (LSD) have none of these difficulties. Like universal translators, an LSD is a great device for moving along a plot. No need to rendezvous with a derelict ship to see if the crew is living, ailing, or dead. No need to explore a whole freaking planet to find suspected castaways or your missing crew mates.
What, exactly, is the LSD detecting -- and across great distances, and often even across vacuum? Intracellular chemical reactions? Implausible (and wouldn't they be different for every species?). Respiration? Body heat (aka infrared)? Perhaps the coldblooded need not apply. Other electromagnetic emissions? Doubtful: It's not like human bodies transmit radio waves. Motion? Nope, the fictional LSD always manages to detect comatose creatures in need of rescue.
None of the above seems remotely plausible. None seems reliable -- even if I could imagine a sensitive enough detector -- to recognize life forms from a never-before-encountered biology.
Which leaves what? A "life force" unknown to today's science that an LSD may detect. IMO, that's medieval superstition, or parapsychological gibberish, or total handwavium.
Bottom line: The life-signs detector is a trope (or LSD, indeed).