I am a UK citizen, resident in Austria. When I was boarding a flight from London to Austria the other day my boarding pass was rejected at the gate ("Unable to board") and I was asked to show my proof of residence and then allowed to board. My interpretation was that they somehow knew how many days I had been in the EU (but not about my residency) and wanted to ensure that I hadn't broken the 90/180 day rule.
So my question is: how does the airline know about my travel history to suspect me of breaking this rule? I fly with BA infrequently but do return to the UK 3-4 times per year for short stays.