system
Given the following post, respond as if you're commenting on Reddit:
human
[General Sci-Fi] How do you scan for life forms? What is the scanner picking up?
gpt
Heat is the main one. It's relatively easy to isolate a single heat signature against a cooler environment, even from a distance, and most forms of life will produce heat as a byproduct of metabolising. Not all, though, and this won't work in hot environments for obvious reasons.

Chemical signatures. A lot of life forms give off chemicals, either as pheromones or as waste products, and these chemicals tend not to be very common in nature outside of environments with life for various reasons. Finding particular concentrations of these chemicals in an area can give you an approximate number of life forms in the area. Certain types of scanners might also be able to look for specific chemicals that are unique to living things as part of their basic makeup.

Neural activity. Anything with a sufficiently complex central nervous system will give off a bioelectric field. It's quite faint but if your scanner is powerful and sensitive enough, you can probably detect it.

Motion. Fairly obvious. Animals tend to move around.

Sound. Again, also obvious. Many living things make sounds, either actively (roaring, sqeaking, etc) or just passively as they move around. If you have a sensitive enough scanner you could also pick up on potential heartbeats, though you also need to be able to eliminate the ambient noise.

Visual. Might seem a bit old fashioned but sometimes the best way to find what you're looking for is to literally look for it. Not manually, of course. The sensor itself would have some sort of algorithm capable of looking at the data and making its own determinations.

Obviously if you know the type of lifeform you're looking for, it's much easier to detect even in an unknown environment, since you know what the signs you need to look for are. And vice versa in that if you don't know what you're looking for, it's fairly easy to miss since not all forms of life will be detectable by any one type of sensor, as well as not every type of sensor will work in every environment. Which is why any sensor worth its salt will use as many different methods of detection as possible.