Semi-siezed slide/occasional squeak
My XLR has sometimes had a slight squeak that I had more-or-less identified as coming from the LR brake.
On changing the rear pads I discovered that one of the two slides was stiff and needed lubrication. Paying closer attention, on other cars I have had with mild squeaks, there has usually been a stiff slide involved somewhere.
I mention this as there have been comments from others that they suffer from squeaky brakes, I assume that they have made sure that the slides are free and fully lubricated.
There are two slides on each calliper (this is on a base model 2004), there is a 15mm headed bolt to remove, the slide has to be held (stopped from rotating) by an 18mm open wrench. If the slide is stiff, you don't need that wrench, this indicates a fault condition which should be rectified, by working the slide out of the calliper (use silicone lubricant). The slide is lubricated with a small quantity of silicone grease, do not get this anywhere near the friction surfaces.