Too Many Variables.
There is no FEDERAL Standard across the Alphabet Soup.
Within various Agencies there are standards
So... ..a little help is needed to actually answer
a. Where did you monitor this ? Hell is a Large Space of Territory
b. What Freq and NAC
c. Thoughts on which Agency ??
And if it was on a federal interop frequency, it could be about any agency...
As others have stated, there is no standard numbering scheme for all federal agency radios. Only certain federal agencies use a sort of standard numbering plan, but just within their own radio domain.