Pull in power inductively from power lines to recharge battery. [0] Use ESP chip with rechargable coin cell [1]. Only turn on chip with no ssid broadcast every hour at some odd minute for a short time. [2] Put/get only URI & decrypt key, main body is on Mega or similar.

Might also make for a neat Geocache.

0. https://hackaday.com/2024/01/27/harvesting-electricity-from-...

1. https://www.instructables.com/Remote-Control-ESP8266-With-Co...

2. https://arduino-esp8266.readthedocs.io/en/latest/esp8266wifi...

May as well just use RFID tags, unless you have a lot of data.