Don't get paranoid with this security stuff.
Look - when You click on some member - You can see his activity - "replying to private message", "posting new post", "creating a new thread". Just use that info in Your scripts and - if any of this flag is active - that automatically log off time should be extended (at least to about two hours - writting article in wiki may take longer time - I believe). If no one of this flag is activated - then after certain time (leave that 0.5h) log this member off. But don't think only about security - think also about frum usage.
