There could be someone, even another you, living in the same spot on parallel universe close in frequency to this one. Electronic hiccups between the two realities, or interference from an outside source could cause certain things happening in the alternate world to sometimes cross over. Kinda like when you hear another radio station coming in on the one you usually listen to due to weather.
And, there's some thing I've heard once saying that the electric signature of someone with a strong emotion, like anger or sadness, gets left behind.
This subject's not really my strong suite. :/
But those who devoutly believe in the paranormal say that if you acknowledge a ghost is there, by talking to it or showing you're afraid, it makes them stronger and more associated with you. Which, I mean, I wouldn't blame them. Imagine being invisible for years and years, and suddenly someone sees you, are starts talking to you. Of course you'd try to stick around them.