Yes, we can track by IP address. They are listed as "guest" or as "Google AdSense Spider". Keep in mind that someone who uses a desktop, a laptop and a smart phone may have a different IP depending on where they "log on" to view the forum. But to answer your question, if someone logs on and/or doesn't log on their IP will show in the "Who's Online" section that is visible to Moderators. If they are using the same device at the same location, their IP will be the same whether or not they are "logged on". It may also be visible, at least in part, to members as well.
ADDED: Members can also "log on" in the invisible mode. To other members, they do not appear "online" but to moderators, they do appear "online" with a red symbol beside their member name, indicating that they are "online and invisible".... So, you can't hide from the moderators