I've found AB logs me out if I've not done anything for a while - happened while trying to compose a particularly lengthy and complicated reply, got logged out, lost what I'd written, had to re-write it - and then it wouldn't submit it because it was too long!
So - to answer the question, I suspect the security implications aren't serious because it automatically logs you out anyway.
In reverse order: Kit, sorry about that! You've discovered that, as with the warning to log out, there is an automatic cut-off if you remain inactive on the site for too long. In both cases, to prevent misuse of your log-in/ username on this site. Hopefully not a great security threat, and, to answer the first question, there are no further implications concerning your online security or privacy. - AB Editor