Ground issue

I am working on Toyota corolla 2005. The battery drains quickly. I did parasitic test but nothing. I switched off all electronic, engine off and key removed. With my multi meter turned to Ampere reading and the probes connected to battery negative terminal and negative connector. I got 0.01A.(as if no drain)
With the same connection I turned my multimeter to DC volts I was getting 13.5V surprisingly. I then knew that there was a problem with ground.
How do I trace out where the problem lies and what is draining the battery?