Ann: Structural Monitoring Systems Investor Webinar, page-108

Actually it can with SMS sensors and that is the issue. Loads on areas of the aircraft change from dynamic flight versus static parked. These loads can close small cracks and make them undetectable by SMS sensors), possibly not even with eddy current methods.

With regard to monitoring in the air and dynamic flight movement versus monitoring on the ground and static. Several issues with monitoring in the air. Currently all the parts SMS attaches to the aircraft are passive(no power needed).
Moving the PM200 to become an onboard piece of equipment would require the PM200 pump to be certified for in flight operation. Big problem, you think getting the sensors approved has been hard, try the PM200.

Next issue, all the pipes that run from each sensor would need to be run from the crack area to the installed (PM200). Current practice is to keep these very short as they are delicate (very small plastic) pipes and are easy to pinch(bad).

Last issue(I will raise at this time), sensors on pressurized surfaces may not work properly, potential for false readings(PM200 was not designed for that).