[phaker]

You were powering it through the protection diodes.

Some 10-15 years ago someone built dirt simple radio tags this way. Just a microcontroller, with a capacitor and an antenna trace connected to some io pin. I loved that hack.

[myself248]

https://scanlime.org/2008/09/using-an-avr-as-an-rfid-tag/

[beachy]

Incredible hackery.