I'm sorry to say I'm pretty sure the stroller and the contents are long gone. There are thieves everywhere including Disney and they know people feel safe leaving valuables in strollers. All they have to do is grab a stroller, walk away, and see what they got later. If they don't want what is there, it may get abandoned but I wouldn't hold out hope.
I'm sure there are also folks who do take things out of convenience. Have a tired kid and it is raining? Grab a stroller to use and then ditch it later.
It sucks when this happens. Glad you got new autograph books.