Interesting stuff...
Detention center doesn't necessarily mean jail. Based on the Harris County Appraisal District map and the way the blocks are subdivided, it could be any variety of detention center applications; from Juveniles to immigrants.
However, based on the property status, I am guessing those plans were long vacated; there are a number of issues with this property that will make it hard to develop as has been pointed out; in addition to the fact that homes are being built / sold on property that is adjacent to this; I would suspect that there would be real estate disclosure laws that would put the developers at great risk; on top of other considerations.
It was clear in the Bridges development propaganda that this was not included - even though it was clearly present on the HCAD schematics.
Might be more info coming.