I have also had experiences of getting faint shadowy FRER lines, or even pink pigmented FRER lines with a beta < 1 miU/mL.
So personally I've stopped getting excited about fainties on FRERs, BUT... every true line starts as a faint line at some point, so it certainly could be the start of something! I might just look for clear progression before I got excited about a FRER fainty. Hope this helps and FX this is it for you!!!!