I have a 2006 Porsche 911 Cab. The "airbag warning light just went on - what does this usually mean? The airbag has never been deployed.

an electrical wire is loose it will come on and go off

AWS is under the passenger seat. I would suggest checking all the electrical connectors are fully seated. You mentioned you had a water bottle under the seat. It could be something as simple as a loose connector.

Also try here: http://rennlist.com/forums/997-forum/740296-airbag-light-2006-997-1-a.html

It could be a number of issues. The most common problem is low battery voltage. Sure the passenger seat sensor (AWS) is a problem as well but not quite as common. The bottom line is that you need to read the codes from the airbag module otherwise it is all guess work. You can read the codes yourself with a airbag scanner. It will be cheaper than taking it to the dealer. Here is a great article to help you diagnose Porsche airbag light yourself: https://www.youcanic.com/wiki/porsche-airbag-light-problems