Earlier quoted context omitted.
Especially a ballon filled with reflective spinning cube, aka Radar reflector. This theory makes a lot of sense and would imply perhaps an adversary has been spoofing advanced radar systems. edit: Also reports of underwater disturbances would be consistent with the concept of these being launched by submarines and ... maybe retrieved by them.
During these incidents, there were US subs nearby, it's unlikely they wouldn't have picked up foreign subs.
https://www.thedrive.com/the-war-zone/21662/swedens-notoriou...