ESP8266 - 12F High consumption in led blink (with pictures)
The SDK library of the ESP8266 remembers on flash memory the mode, SSID and password of the last WiFi network it connected. At start or by enabling STA mode it tries to connect to that network. The SDK library is always part of the compiled sketch for the esp8266.

The esp8266 WiFi library has some functions to control this:


WiFi.setAutoConnect(false)
WiFi.disconnect(true); 
WiFi.persistent(false)