Any idea about the devices/circuits drawing current from the car battery while car is in off state?

Open the negative terminal of the car battery(keep positive connected with the terminal) in off state(even all accessories also turned off) of the car and measure voltage between battery's negative terminal and the chis(earth) of the car, it gives slightly lesser value of the voltage than the potential difference across the battery in open circuit. That shows some current is being drawn from the battery and hence there is some potential drop.What are such devices/circuits in the car which are causing drainage of the battery?