Well I didn't drive the car at all yesterday other than to pull it out of the garage and then two or three hours later pulled it back into the garage - no warning message either time. However, this morning when I started it to go to work, I got the dreaded "Alarm System Service Required" for maybe 8-10 seconds after starting. Drove it to work which is about 9 miles/20 minutes. Shut it off at work and immediately restarted - no message.

Now I'm starting to wonder....

Are the AA NiCads up to the task?

Is the battery disconnect on the circuit board too far corroded to support a good battery connection?

Is the battery pack now too far (18-24") away from the siren module for it to charge up and feed back properly? (One of the guys on YouTube put the battery pack inside the ECM box and he mounted the siren module just outside the ECM box.) I should have checked the new battery pack voltage at the circuit board connection to see how much line drop I got.

The fact that I don't get the message if the car has recently run but I do get the message if it sits all night makes it sound as though either the battery pack isn't getting fully charged when the car's running or it's draining somehow when the car's at rest. I'll start by disconnecting the battery pack at the bullet connectors and checking the line to the siren for drainage.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)