I'm very interested in this thread. As I have had a similar niggle. It first appeared last spring whilst training (upping the mileage & ascent / descent) for the JNLC. A nagging niggle at the rear of my Left thigh, directly above the knee. At that time it was particularly noticeable whilst laid flat on my back with my legs straight out. Rightly or wrongly (absolutely no medical qualifications or experience) I guessed it was an issue with the tendons at the lower end of the hamstring. Stretching seemed to ease it enough to continue training and a rolled-up pillow under the back of my knee allowed me to sleep on my back with the knee bent, trouble free. Absolutely no problems at all when out cycling - regular 60 milers at an easy pace.
After completing the JNLC at a very steady pace (V65, so 24 hours) in August the issue subsided all through the autumn and Christmas.
However, at the weekly interval session 2 weeks ago, after a particularly enthusiastic effort, it reappeared.
Now the niggle is more noticeable with the knee bent, rather than with it straight. It is not noticeable when cycling seated but makes its presence felt ever so slightly when 'honking' out of the saddle.
A new club member, who is a qualified physio and pilates instructor, is looking to put some suitable exercises together for me.