Any thoughts on this...?
Anything that moves with this pattern frame to frame is either a permanent or temporary fault on the detector array. You see this, or sometimes a black circle or other shape, move around frame to frame in exactly this way. The detector arrays are not usually perfect and single pixels may be defective; also they can be temporarily disrupted by ionising radiation from a cosmic ray or a local, low level, radioactive source. This can also affect quite large groups of pixels or (as is also present on this set) whole columns due to the way the information is read out of the array. Each picture in the set of 4 is slightly shifted in this pattern (a slightly crooked inverted crucifix shape) so the same region of the array is in a different position on each frame.