High number of connections

After hours last night, I got a couple of calls from folks working late 
that they could not connect to Domino on our iSeries.  When I looked at 
it, only two things I saw were out of the ordinary:  The CPU was in the 
high 90s, almost all taken up by the server process (usually there's 
another process that's the culprit when my CPU's burning up) and in the 
Domino console, I could see that there were no transactions taking place 
for the better part of an hour and a half and there were over a thousand 
users connected.  We don't have a thousand users and almost all of them 
were home with their PCs turned off.  For an hour and a half, another six 
to ten "users" would connect every minute. 

I ended up forcing the server down and it hasn't repeated this little 
performance yet today, but does anyone have any idea what might have 
caused these phantom connections?  We had no sign of a packet storm on our 
network, so it doesn't appear to be a case of a mass-mailer virus trying 
to send out -- last time we had one of those, it didn't produce this type 
of result, anyway.

Anyone else see this?  Any idea what it was?  We're on i5/OS (is that the 
right name now?) 5.3 and Domino 6.5.4FP2

Thanks,
Patrick