Re: So, shoot the messenger is still well and alive
"Hmm, AdBlue needs to be topped up (fairly) regularly so is unlikely to be hidden. So I wonder if Ford were also using a similar system to Stellantis - i.e. a hidden fuel additive tank or pouch (Eolys, pat fluid, other names) that the car decides is empty after certain parameters are met (e.g. 100,000 miles travelled / x fuel ups, or somesuch). THat sounds more likely including the price to refill."
This is the answer. Ford used DV4 and DV6 Peugeot/Citroen diesels in the Fiesta, Fusion and Mondeo range in 1.4 and 1.6 capacities, and IIRC they were fitted with an Eolys aftertreement system which as you say needed refilling and recoding with the correct tools.