I had a spot in my image as well, see the picture in my post https://www.eevblog.com/forum/testgear/flir-e4-thermal-imaging-camera-teardown/msg329239/#msg329239
I saw that image the time you posted it, but in my memory the spot was somewhat bigger, so I didn't bother searching for your post. After now seeing it again and realize it's the same size and shape, it convinced me to play around a little. So I used a duster ball to blow air in some of the housings cavities, in the hope of again moving something inside. With success, after this "treatment" a 2nd spot with the same appearance showed up on my image

So any grain of dust on the sensor may create a similar looking disturbance, with it's thermal intensity depending on the size of this dust particle.
After this I evaluated my options in cleaning the sensor surface using Mike's teardown video. Then one led to the other and in the end I had my unit disassembled, the sensor dusted and everything reassembled again.
I'm now spot free, so my sensor and lens are OK

To keep things exciting, I now have some offset between my thermal and the visual image, making MSX look a little odd on greater distances.
Anyone knows where to adjust this? Is this done in software or do I have to dismantle my E4+ again?