The specification and drawings present methodology for modularly expanding sensing capabilities of a luminaire or a luminaire mounted sensing system by adding replaceable exterior sensor module(s) in a desired position in a vicinity of an existing (legacy) luminaire or a luminaire system for both inside and outside applications. For outside applications, this modular expansion may be performed while maintaining weathertight sealing integrity, e.g., waterproofing of the luminaire-sensor system/apparatus including exterior housing, exterior electrical connections, cables and any other exposed components. The embodiments described herein can provide diverse sensor capability and future expansion needs for luminaire mounted sensing systems and for processing sensor signals.