Of all the places I've ever nvestigated, it's Hull House that generates the most false positives - photos that LOOK like a ghost, but actually aren't, even though some of them are really, really cool. Here are a couple that have been taken on tours that I've run over the years, representing what are probably the two most common of them:
1. The "Monk" - a ghostly figure that looks kinda like a guy in a hooded robe. Monks at Hull House are pretty well known:
2. The "feminine form" apparition. Pictures of a little girl were common a couple of years ago, but, like the rest of the hauntings there, they've tapered off. Here's a cool one, which could also probably pass for a monk:
I'm not saying Hull House isn't haunted - we used to get some GREAT shots there, though it hasn't been all that active in a while, leading us to avoid it on most tours. But it's also THE easiest place I know of to get a false positive, and these ones aren't ghosts.
In fact, they're probably both the same thing: ears. Reflections of people's ears in the window through which the picture is taken. The top one I'm less sure about, but that's definitely the case in the bottom one. I've gotten at least one or two good "little girl" pictures at Hull House that I've yet to explain away, but every monk on the stairs I've ever seen has turned out to be an ear in the end. In many (including one of these, in its un-cropped incarnation), if you look closely, you can also see the nose, which is a dead giveaway.
I'm not saying the place isn't haunted - on an "active" night, it can be the spookiest place in town - but there's a LOT of crap going around about the place that ghost hunters ought to ignore. There's no abortion graveyard on the grounds, no headless ghost that'll follow you home, and there was never actually a devil baby (though there was actually RUMORED to be one in 1912 or 1913 - I forget which. See the tag to find an earlier entry with a link to an article Jane Addams wrote on it). But there DOES seem to be something weird about the place, and we've dug up some information on it for the upcoming Weird Chicago book that hasn't been widely known before!