ID Scan will only receive the talkgroups that you have programmed, providing that the talkgroup is not avoided (locked out) and it is not in a system or department that is avoided.
ID Search, on the other hand, will receive any active talkgroup on the system, providing (once again) that the talkgroup is not avoided (locked out) and it is not in a system or department that is avoided.
If it is a talkgroup that you have programmed, you would see whatever text tag you have associated with it. If it is active, but not programmed (and also, again, not avoided in the ways mentioned) you would only see the TGID. Then, if it appears to be used for something that you want to listen to, you can add it to the system, and once you determine agency & usage, add the appropriate text tag.