Session stays open many hours(!?)

[Theodore Hope]

I've just installed the latest stable horde/imp and am
trying it out.  I left open an imp session last night,
and when I came to the desk again today the session was
still active.  In other words, I could read msgs as
though I had recently logged on, but it had been about
14 hours since I touched that browser.

The components are these (running under BSDI 4.1):
 horde-1.2.4
 imp-2.2.4
 Apache-1.3.17
 PHP-3.0.18
 IMAP-2000c
 Postgresql-7.0.3

The setup was done "manually" (not with a script or setup.php3 or
anything like that).  Everything appears to work just fine.

I didn't manually configure any "timeout" values at all, and
the "lib/horde.lib" file contains "$this->session_timeout = 300;".

The only thing I can think of is that I did start an imp session
with the same mail user about an hour ago, but from another
machine/browser.  (I wouldn't think that the phplib+imp session
stuff would think the two browsers were the same person, since
it's unique cookie-based, right?)

Any thoughts?  Do sessions really last much longer than 300 seconds
(unless explicitly logged off)?

The client browser is Netscape 4.75 (under BSDI 4.x, X-windos).

Thanks.
 -T.H.