Arduino leonardo analog input powered the board?

Hi everyone, when I am using the Arduino Leonardo to read the voltage of a battery (vol ranges from 2~4V depends on charge state), I noticed that even I didn't plug in the power supply to the Arduino, no connection from the USB or the power jack, the connections are only battery leads, one to the A0 analog input and another lead to the GND pin. I thought there should be no current now (since internal impedance is high), but I could see that the LED ON and L are on with some weak light. Does that mean the board is power somewhat from the analog input from the battery?
I wanna minimize the battery energy loss on the circuit and read its voltage. How should I make sure the battery is not powering the board?

The IO pins have diode from the pin to VCC and from the pin to Gnd to prevent the voltage on the pin from exceeding VCC + 0.5V and Gnd -0.5V.
So if your processor is not powered, and apply voltage to pin, current flows thru the diode on the pin and can appear to phantom power other things connected to Vcc.
If too much current flows, the diode can fail and not provide protection for that pin.

So: Don't connect active signals to unpowered processor chips.