The only time I've ever even heard Lexi growl was in one of my college townhouse apartments. I swear to God, the place was haunted. I'd hear all sorts of weird noises when no one was home - like when the entire building was gone on Christmas break, etc. It always gave me such an unsettled feeling. Anyway, one day I was there alone during the holiday to get some of my stuff (literally there was no one in our 4 townhouse unit)… I was in my room with the door shut when I heard someone putting away the dishes. Clink, clink... clink, clink... like when you put the glasses away. Not thinking, I just opened the door to see which of my roomies had come home. And of course I opened the door to no one. I walked in the kitchen and had almost opened the cabinet to see if the glass I had put in there had fallen over or something when I realized I had not been in the kitchen. Um....... I pretty much froze and freaked out.
I quickly ran back into my room and shut the door. That's when Lexi (only about 9 months old at the time) started growling at the crack under the door, her hackles straight up. I seriously got chills. I had never seen her do this. I finished what I was doing and opened the door slowly as I didn't know what would be on the other side. She slowly turned the corner, all hackles raised, and then ferociously growled very pointedly towards the end of the long hallway I had to walk down to get out. I could see all the way to the door and there was clearly nothing there, but I sat there for a moment frozen trying to figure out what she was looking at… because she was clearly focused on something. She didn't move; I didn't move. And this was in the middle of a bright sunshiny day!! Very few times have I ever been so terrified.
I have never seen Lexi growl since. Ever. (grunts and snarks, yes, but never growling) That was such a weird experience. I'm not one to believe in ghosts and stuff, but if there were ever a time...that'd be it!