In Atomic Blonde Lorraine and Delphine are in Lorraine's hotel room. Delphine says something along the lines of "I need to tell you something about your friend Percival..." but Lorraine cuts her off. Lorraine turns on the radio so that (I presume) any bugs in the room can't hear what Delphine whispers in Lorraine's ear. The audience doesn't hear and it isn't revealed later in the movie as far as I can tell.
What does Delphine whisper into Lorraine's ear about Percival?