I keep user manuals. Sadly, I don't always READ them. From the STX3 manual:Good golly you are on it. Doh!
Never mind folks. Nothing to see here. Darn thing has been correct for 6 years running, it is just the USER that needs recalibrating. I will confirm this in a bit, but this would account for what I was seeing yesterday.
Thanks @piranhaman00
"Open the daylight plate and apply one or two drops of distilled water on to the surface of the prism. Hold the prism at an angle close to parallel with the floor so the distilled water will not run off of the prism."