Welcome to TMC, TheD!
I admit that I am biased because I firmly believe there is no "Model S Refresh" anytime soon (i.e. this year). There might be incremental changes, like the new 19" Slipstream wheels or the integrated console, but overall, no, no refresh is "imminent". People have been saying this for more than a year now, and yet nothing.
The CPO inventory you are now seeing listed are not CPOs, but "Inventory" (new) cars. Due to their low (50) mileage, they are most likely cars pre-built (not to a customer spec) in order to have an inventory of cars people can buy without a production delay. They are not "showroom" cars. If Tesla has a temporary overcapacity situation, it doesn't make sense for them the stop or slow down the production line, so cars will continue to be pumped out and sold "off the lot" like any other car dealer. These cars are being distributed to all the Tesla Sales centers across the country.
Tesla is still a "small" (relative) company. They are focusing on ramping up the Model X. Finalizing the Model 3 and getting that into production. Building out the Gigafactory. Thinking about whatever the Model Y will be (Roadster V2 or a pickup truck??). I think the Model S "Refresh" exists, but it's way way down on their list of priorities. I don't think we'll see what can be called a "Refresh" until slightly before the Model 3 is launched, and that would be to minimize any cannibalization between the two models.
But this won't stop lots of people preaching the Model S Refresh is "just around the corner".