Trivial "Hide coffin" unlock appears upon spawning as vampire

[Studenterhue]

One time when I spawned as vamp, it said I had suddenly gained Hide Coffin, except I definitely hadn't. I hadn't sucked any blood, and I didn't see Hide Coffin as one of by abilities. In fact, when I got the 300 blood needed to actually unlock it, the unlock message appeared again. I didn't use it, but the ability icon did show up.