mysteriously expiring sessions

[Pottinger, Hardy J.]

Hi, I've been chasing this issue for a while, I have users that occasionally walk away from their computers (bathroom breaks, reload their coffee, *meetings*, the usual) and they grumpily complain to me when they have to re-log in, so I've made changes to our shibboleth configuration to accommodate them (I think). However, they still report that their sessions are timing out, so, I have turned on debug logging and am trying to figure out what's happening. Today a user gave me enough detail to actually look in the logs and find her session, here's what I've found:

2016-02-04 09:04:04 DEBUG Shibboleth.SessionCache [994]: creating new session
2016-02-04 09:04:04 DEBUG Shibboleth.SessionCache [994]: storing new session...
2016-02-04 09:04:04 DEBUG XMLTooling.StorageService [994]: inserted record (session) in context (_9b6f04da7f13d869e3f79338fe964d01) with expiration (1454684644)

OK, great, she has logged in... I scan down for any mentions of that session, I see a few, here's the last one:

2016-02-04 09:07:45 DEBUG XMLTooling.StorageService [978]: updated expiration of valid records in context (_9b6f04da7f13d869e3f79338fe964d01) to (1454684865)
2016-02-04 09:08:27 DEBUG Shibboleth.Listener [983]: dispatching message (touch::StorageService::SessionCache)

that Unix timestamp on that line (for the expiration) converts to: Fri, 05 Feb 2016 15:07:45 GMT, which, converting to CST: Fri, 05 Feb 2016 09:07:45 CST, is actually still valid... so... something else happened to this session.

Scanning down a bit, I can see where she re-logs in and gets a new session

2016-02-04 12:39:10 DEBUG Shibboleth.SSO.SAML2 [1052]: decrypted Assertion...

So, sometime between 2016-02-04 09:08:27 and 2016-02-04 12:39:10, her session information expired, and looking at the log files, the only lines mentioning expiring sessions are at 10:24 and 11:24. 

2016-02-04 10:24:02 INFO XMLTooling.StorageService : purged 6 expired record(s) from storage
2016-02-04 11:24:02 INFO XMLTooling.StorageService : purged 1 expired record(s) from storage

Has anyone else observed Shibboleth incorrectly expiring session information from the cache? Is there any configuration option I should be looking at that would prevent this from happening? Thanks!

--Hardy