No, this isn't a Scooby Doo mystery, but rather it's a Disney one. We've often driven past the end of the road to the Bonnet Creek resort and wondered why there is a non Disney resort in WDW and why it doesn't feature on any of the signs. I even googled it when we were there last time to see if I could figure it out, but all I found were lots of sites that would let you book a stay there, but no explanation.
DVC News has come through with a link to a website called Yesterland, which I hadn't heard of before, and definitely looks worthy of a further look at some point. But anyway, the link is to an article all about Bonnet Creek. It turns out that a parcel of land, just under 500 acres in size, was never purchased by the Disney company when they embarked on their land grab in the early sixties. It had already been acquired by someone else, and Disney either couldn't track them down or they didn't want to sell. Either way around they have clearly done spectacularly well by holding on to this land, as it has no doubt skyrocketed in value over the years.
(Now that I look again at the map of WDW in our latest Unofficial Guide, you can clearly see a cut out of land, just to the west of I4 and to the south of Epcot Centre Drive which is not shaded in as Disney territory.)
Currently it is home to the Bonnet Creek timeshare resort, and there are a couple more hotels being developed on the site, including a Waldorf Astoria, which is due to open this October. I think we may well have to pay it a visit for a look around and perhaps a meal when we are there at the end of the year.