It's been two years since I've cleared customs or been to Signature MIA but, at that time, UPS had hogged the Northwest Corner, customs had the next ramp over. The other half of the customs ramp is transient. At the back of that ramp is Millon (Mie-yone) Air, which, I believe transports horses. Signature is after that, and then, as you said, a bunch of Maintenance hangars. In the group of Maintenance hangars is Miami Air and Pan Am flight training.
Signature, Miami Air and Pan Am may have websites with maps for driving directions, which may include the surrounding buildings.........maybe?.?
This may provide a little insight......maybe?.?