In Peace Talks when Austri is asking for entry passwords, Harry notes about Thomas: “Because he has nothing better to do with his time except memorise random passphrases”.

JB likes to drop little hints that initially just read as an unremarkable line. Thomas may likely emerge from Demonreach as a different being, but what else will he emerge with, especially with the British mystery man whispering in his ear? Thomas really won’t have anything better to do with his time except heal and listen. What is he listening to and what is he learning? Will it be information that helps Harry in the future / past?