Portlander can probably speak more to this but I believe the numbered buildings were part of a proposed development that included refurbishment of two existing structures (the Fore Street building and the former Hannaford distribution warehouse, now Memic), and also a planned commercial use for the block that is now the Portland Harbor Hotel (formerly a parking garage before the hotel was built).
The 150' standalone building dates to 2007 and was supposed to house legal firm Pierce Atwood, which eventually moved to the Cumberland Cold Storage building on Commercial Street. The offices would have been built as you suspected behind the police station, next to the courthouse, and would have also housed the offices of Cumberland County government.