In many games and sci-fi worlds, there's a kind of "biological radar" that somehow knows where nearby organisms are. How would such a “radar” be possible? I'm looking for a way to detect all large organisms, including plants and cold-blooded animals that don't show up on infra-red. Ideally, this would work underwater as well, detecting things like jellyfish. For the purposes of this question, microbes can be ignored- although a clever explanation for why this would be the case would be awesome.
This should be something possible on Earth with minimal hand-waving, so you don't have to worry about detecting aliens or beings that don't obey our current biological understanding of macroflora/macrofauna.
I’m hoping for something handheld, like a Star Trek tricorder, and which doesn’t require an extensive array or satellite/ship based mechanism.
EDIT: I am not looking for a way to use current radar to detect biology. I’m asking how sci-fi civilizations would be able to detect life forms using something like radar.
For example, from Star Trek TNG: "Captain, I am detecting life readings from the planet's surface". What would this instrument be "reading"?