I've had this tree for about 7 months. I watered it according to the nursery instructions...water until it runs into the tray then wait until it is dry for about 6 inches on the top. I also fertilized with winter mix (from the nursery) until Spring, then with growth mix (also from the nursery). Until recently there was good growth and quite a few flowers (although as soon as they started to become lemons they dropped off). In the past month plus it's started dropping leaves and when I used a moisture probe it showed very moist even though it felt dry to the touch more than six inches down. I tried mixing some orchid mulch into the area around the roots and haven't watered for quite a long time. Nothing works so far and I think there may be mold mites infesting it. I'm really discouraged. I've done everything I was supposed to do (according to the nursery) and even some suggestions from this site. I'm afraid I'm going to lose the tree.
The most common cause of citrus dropping leaves is root rot due to over-watering, even temporarily. The pic does not show how your pot is mounted, but ideally you will have the pot up on pebbles so that any excess water gets immediately drained away from the pot. Sitting in water would be very bad for lemon. From your description you have been careful to avoid this.
Leaf drop can also happen when there is a sudden temperature change. Note how the branch leaning away from the window has leaves. From the heater radiator visible against the wall you have central heating which indicates that the window can experience sudden cold. In really cold weather and sudden changes pull the pot away from the window towards the centre of the room where the temperature changes will be less dramatic. A brightly lit window due to direct sun does not necessarily imply a warm window.
PS: check the window/door for leaks. Everything looks impressively clean and new but a cold breeze that filters in rapidly would be a shock close to the window even if not felt in the centre of the room. Do a smoke test rather like that performed while doing an energy audit.
It's not common in these questions, but you might be under watering. You say you use a moisture metre. Those are not accurate. That is why you are feeling and seeing something different than what it is telling you. Throw out your moisture metre. Use a bamboo skewer instead. Insert the skewer into the soil. Wait about 30 mins. Pull it out and check it for moisture. Look at it and feel it. It it tell if there is moisture further down. Depending on the depth of your pot, 6" before watering might be too deep. The bamboo skewer will tell you what it is like by the roots. The only place that is important.
Usually, citrus that are dry, leaves will curl then become crispy. Your leaves just dropped. I still think you may have been underwatering, but you may have been overwatering. It's impossible to tell.
I think you should immediately invest in a pack of bamboo skewer for barbecuing. Then give your plant a check for problems with moisture.
Other stressors might be pest. Have you inspected the leaves and stems for pest. Spider mite love to feed on citrus leaves. Do you regularly, once a month, rinse off the leaves with a hand held mister or in the shower? If not it is something to think about. You did not show us a leaf that has fallen, so I am not sure if you have pest attacking your plant, but it might be that stress.
Or combined stressors of a watering issue, pest and someone opened the french doors by the tree. Colin's answer about temperature is sound. You say you rarely open the doors, but do admit to opening those doors on occasion. Is there another door or a radiator near-by?