I do not bother telling them I have a medical condition unless they ask - it is personal and none of their business.Bearing in mind anyone can purchase a Libre, wear it and pose as having a medical condition, so I always show my letter from Hospital / NHS to the first Security Officer I see, this may help of the bat!
I have been wearing Libre through airport scanners many many times a year for about 5 years and never had any issues.
I have also put unused ones through the x-ray and never had an issue.
If it puts you at ease, there is not problem telling the security staff but you do not have to.