They dropped 1/2 conduit from the ceiling about 2-3', then mounted the sensors
That's a great idea!
Mount the Occ. Sensors to 4s Boxes (with a 1 Gang Ring or equal mount), and suspend the Sensor/Box assembly from a Conduit type suspension means (like the way a Ceiling Fan is suspended on a GRC Nipple).
Proper supporting for a "Typical" conduit run, along with seismic activity (if applicable) should be included too.
As for the Occupancy Detectors, the Company "Watt Stopper" has a line of Ultrasonic Occupancy Detectors that might be perfect for your application.
There are "Dual Receiver" (Microphone) types with 1000 Ft² and 2000 Ft² ranges, which would be ideal for the "Central" areas of the Aisles, and "Single Receiver" type with 500 Ft² range - which would work great on the "Ends" of each Shelving Aisle.
These Occ. Detectors may be fine tweeked for delay off time and sensitivity.
They are used in conjunction with Lighting Contactors - known as "Power Packs", but any Contactor with a low voltage coil may be used - provided you have some type of low voltage power supply to run the detectors (24 VDC I think...).
Let me know if you would like additional information.