|Good Morning Guys,
Yes you can determine how much gas has been lost with a thermal camera. You need to make the detector really work and beware of any reflections.
You will always get a cooler area in the centre of a double glazed unit. This is due to it being made either flat or at an angle and is where the cavity is thinner due to glass bow. This is exacerbated in cold weather and in some cases the glass will touch in small gap units causing a dew spot inside the unit most common when the spacer is either 6mmor 8mm.
Sorry I digress, to identify the gas loss first you need to know how old the unit is ( over 5 years, there is a possibility that all the gas has leached out. You then need to get a goo difference inside and out 10-15degrees C. The next is to find a same unit as a control sample. Then you are able to take a good thermal image. However, please be aware that there is also the element of what is the glass unit made of, are there any metal coatings? What gas has been used.
As for being cold near a pair of patio doors, there is always the element of convection or stacking where cold air gets hot and rises past the glass as it gets hot. This gives two noticeable affects the first is that there is a draft in extreme cases and the second the glass and air near it is cooler that the rest of the room.
Hope this helps, a bit long winded but having been carrying this sort of investigation for the past few years I have found may of the pit falls. It is not an exact science.