Could be water vapor, but also any oil or lubricant. It is, after all, an engine. Wonder if any other members here know more than me, though (as I readily stipulate that my comments here are not terribly helpful).
Awaiting push-out at Indianapolis years back, sitting in a 720 (like 707, but smaller), a worker came out, got up on the wing, opened a small cover, and poured
ONE QUART of oil into an engine! When I got back to Chicago, asking my boss, who was likely the most capable and knowledgeable Mechanical Engineer I've ever known, he revealed those engines' sumps only hold 2 gallons of lubricating oil, and are certified to fly with only 2 quarts remaining!
Unbelievable. Turbojet engines just don't "use" oil. Leak, maybe. imp