It's a feature in UBB that was previously turned off (except for a short period of a few days when it was turned on then turned back off). Every time anyone makes a post with the feature turned on, their IP address is logged.

Note, that it can only log IP addresses after it is turned on, so even though it might show every thread in a post as being logged, they aren't if the post was made before logging was turned on.

Also, the reason some posts don't have it is no addition to a thread has been made since it was turned on.