XLR doesn't have a PCM. BCM is definately a good first candidate to suspect, but intermittent glitches, --like timing errors that trigger error messages without no codes, are very hard to pin down unless spare modules are available to swap in/out, which GM discourages.
Complexity is a double-edged sword. XLR's architecture takes a lot of the traditional labor out of the troubleshooting process, but when the normal paths don't apply/work, it takes a detailed level of system knowledge and lots of experience to root out the problem.
In a case like this, it would be helpful to use a logic protocol analyzer to monitor the nodes on each bus to determine which module is setting the flag that creates the message. You would also need firmware documentation to make it all comprehensible. This level of troubleshooting is way beyond what a dealership is capable of, which is why module swapping (what is commonly referred to as Easter-egging) is relatively quick, and if done logically, yields reasonably fast results when traditional methods fail.
It could be something as simple (but often hard to find) as a dirty pin in a connector, or broken strands in a conductor changing the resistance of the bus as well.
Keep us posted!
CC
