My boyfriend has had an African Grey for the past 13 or 14 years. We have been dating for over 9 years, so I've been around him a lot. I also take him to his avian vet appointments. Murphy, the Grey, is very temperamental. Any little change in his daily routine can start him feather plucking. Move a couch, or change the curtains and he gets stressed. In my opinion, it would be really hard to find exactly what upset Scarlet. I wouldn't worry too much, especially since the vet found no physical problems causing the picking and because her feathers are growing back.

Make sure she isn't on an all seed diet. Greys LOVE pizza, but don't give that to her too much. Scarlet can and should be eating a lot of veggies and fruit, but she doesn't need to be limited to those foods. Anything aside from avocados, alcohol, and chocolate is ok for her. Watch her salt and fat intake and take the pits out of fruits that have them. Also make sure Scarlet is getting plenty of calcium, especially because she is a female. You might know all that already (I noticed your profile with 21 birds ), but diet can be a main problem for birds. Hope that helps or at least makes you worry a little less