Guard-ring circuit is designed aroung high sensitive (low current high impedance inputs). What is the technical impact if a guard-ring circutry is fully masked versus unmask ?
The solder masks generally used to reduce moisture infiltration to the PCB (thus can prevent leakage in some degree), but there may be surface-charge problems when using large area solder-mask. So the better approach is to use bare-copper guard area around sensitive nodes. Using solder or plating the guard with gold or tin to prevent oxidation. The guard trace is to provide a path for the leakage to the low impedance node, so they should not be covered by solder mask.
And There are some similar discuss: http://www.eevblog.com/forum/reviews/6632a-teardown/
... you can clearly see the guard traces ( silver ) . the reason for removing the soldermask over the guard is to remove that pathway as well. Soldermask has a specific dielectric constant that is worse than air. By removing the soldermask you make a lower leakage capacitance across the guard ring.
Notice on the left bottom pin that there is again a guard ring present. For some reason they forgot to open the soldermask ( i looked carefully and it is a guard ring ... )