Hi saky - is it possible that there's a bit more carb creep than you're aware of? I certainly found that in my first couple of years after starting low carb i was very vigilant about everything I ate. That relaxed a bit as time went on and I got to target weight.
It can mean that - for example- I have two low carb buns in a day instead of the usual one: not highly significant except that it's an additional 6g carb and if it's been a 24/25g day already from other sources I can find myself unexpectedly out of ketosis, or with a higher than expected reading. And something like a vegetable curry can have unexpected effects given that the carb content of veg isn't always that predictable - tomatos are all over the place in terms of how much carb they have.
Sometimes a short fast - 24 hrs, 8pm to 8pm - can give your system a bit of a bump back to where it was, as can a totally zero carb day. Or it's possible that your liver might have had a few ideas of its own about where your BG should be based on stress or illness or activity, and upped your circulating glucose.