Forensics is the work itself: using scientific tests to solve crimes. You send a sample to forensics; she works in forensics; the forensics came back today. It is the name of the field, like a subject at school, even though it looks plural. Though actually, in some American schools “forensics” also means the debate club — that is a different, older “speaking in public” sense, closer to the old forum. Here we mean the crime-lab kind. People treat the word as a lump: they rarely say “a forensic” for the science, and “forensic” by itself is usually the describing word.