I was going to suggest most of the things you have already mentioned that you've checked, so it is a bit odd. It's either a wiring problem somewhere along the way to the instrument cluster, or that the actual gauge in the instrument cluster has gone faulty. I know it does not make any sense, as you have not been anywhere near it, but from what you have checked already, this is the only thing left to check. You could check for a reading with your multimeter at the actual connector that plugs in the instrument cluster and see if you are getting a signal. This way, you can narrow down where the problem might be.
For wiring diagrams, see here
2000_S40_V40.pdf
2001_S40_V40.pdf
2002_S40_V40.pdf
2003_S40_V40.pdf
2004_S40_V40.pdf