peltier driver using AMP OP and MOSFET
If you want to control the current in the Peltier device it should be in the drain  circuit of the mosfet and a resistor in the source (where you currently have the peltier device).

Your circuit is currently feeding back the voltage across the Peltier device.

Also if there is any temperature difference across the Peltier device it will generate voltage even when unpowered - that is probably what you are seeing right now.