Another place that isn't technically abandoned, although this time I was aware of this in advance. This former high school was built in 1908, but has been used as a community center since the 1950's. It closed in 2014 due to a water leak, and is currently undergoing asbestos abatement before being reincarnated yet again as housing.
This location is laughably close to where I live, but I've always thought that it was still in use. When it started showing the telltale signs of abatement and renovation, I thought that it would provide the best chance I'd ever get to conduct my own unguided tour. I was on pretty on edge as I entered; the power is still on, the only entry requires using the elevator, and each elevator is covered by a camera, with motion detectors at the end of every hall. This turned out to be a hollow threat, though, as missing ceiling tiles showed that the cables for these instruments all led to a "security office" that had been completely gutted down to the bare walls. Apart from the gym, which is pretty much unchanged since its first renovation in the 50's, the interior is sorta bland, but enough remnants of its former lives still remain to warrant documentation.
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Thanks for looking!