"Unit neutralised" sounds like a pretty standard thing to say about having encountered a potential threat, then rendering it unable to threaten. Presumably Kate introduced herself to the sheriff at some point to establish authority, and if there was a need to report in, then that presumably meant that the Zygon had reported that someone was there in the first place. "UNIT neutralised" is also, therefore, a reasonable assumption to make about what needs to be said in this case.