Coherence will cache UCM Nodes. It means that will cache the content information and will prevent going to search again to UCM every time.
Another test to check coherence is:http://docs.oracle.com/cd/E29542_01/webcenter.1111/e27739/jpsdg_content_presenter.htm#CHDHHFCE
Your idc log files aren't showing GET_SEARCH_RESULTS, GET_FILE or queries to retrieve the content information. It means that the content is served by Coherence instead of UCM.
Which version of WebCenter Portal are you using?
It's like Daniel says. If coherence wasn't properly working you would see much more services in the audit. The info you provide seems fine.
I would also recommend following my article on how to optimize the cache settings in coherence: http://www.ateam-oracle.com/optimizing-content-cache-performance-in-webcenter-portal/
Especially the part about setting the expiry-delay to 0 is important for optimized performance.