I forwarded the photo to an distribution engineer friend of mine at a
nearby power utility. He suggested it could be a remote meter reading
transponder repeater. The technology has numerous ways of doing it, and
low power radio modems are used in some demand metering systems where
individual phone lines are not practical. The transponder gets its power
via the power tap inserted between the lamp body and the light
controlled switch. The nearby low power transponders all report into
their local transponder which in turn relays the reports to a central
gathering station. That's how the system works. Whether or not that is
what it is is yet to be verified.

The antenna's element and coil dimensions look too big to be 2.4GHz. It
does however look right for ISM.