Warehouses must maintain a number of spatial constraints concerning the safe handling of hazardous materials. In this paper, we address the problem of automated monitoring of spatial constraints as pallets are moved on forklifts by human operators. We propose using small, self-contained localizing sensor packages mounted on pallets. We consider two architectures: 1) a global architecture where sensors are mounted on pallets and on the warehouse ceiling, and 2) a local architecture where sensors are located only on the pallets. We formally model the problem for a warehouse environment where sensors can be installed on the pallets or on the ceiling. We also report on preliminary experimental results on the accuracy of constraint violation detection as a function of inter-sensor distance and angle.