Sounds to me like you're reviewing a Trunking Discovery session and have (had) "Compare to Database" turned on (during the discovery session). The scanner will list talkgroups as "known" when they come up in discovery and are already in the scanner's internal memory. What "known" means in this case is, "I've already got this talkgroup, so I'm skipping it and checking for others".
Discovery Mode is used for finding new talkgroups on a system. It will listen (and record) talkgroups for a period of time (relating to the timeout timers you set up in the session settings) so that you can listen later and try to figure out what the talkgroups are used for. If you turn "Compare to database" ON, it will do as I said above. If you turn "Compare to database" OFF, it will treat every talkgroup it comes across as undiscovered and record it as new.
If you haven't already, browse to the Easier to Read Manual for the HP2. It'll help out a lot. I've owned all kinds of scanners over the years including several HPs and I still refer to the Easier to Read Manuals from time to time to look something up.