Passport check at Caen on return to France from UK

Hello all, having visited our daughter back in the UK last week we returned by boat to Caen (a trip we have done thousands of times !) we presented both our passports at the Immigration hut as usual, OH was scanned rapidly,no problems at all. Mine was another matter ,the Officer scanned ,looked at the monitor, puzzled, pushed a keyboard button again & a page flashed up (we could see the display clearly ) showing my current passport page details as usual , but alongside this to the right were 2 grey boxes,one above the other with the Capital letters E(topbox) and F (lower Box). The Officer continued to scrutinize the monitor,she did return both passports wishing us ‘bon soiree’ to our departing statement, however she continued to closely scrutinize the passport page even as we drove away !! What the heck was that all about ?? Never had an issue in the 18 yrs I’ve ping-ponged back & forth ,now suddenly this. What do the letters stand for ? if it’s a check or a ‘flag’ how would I find out ,any realistic ,sensible suggestions based on KNOWLEDGE or experience would be really helpful,as we’re somewhat concerned . Bon soiree.

You should just have asked them. Unless there is a member of the police aux frontières handy you are unlikely to get a proper explanation.