Short in wiring?

[alborz]

I looked for a short in wiring harness and watched a video on youtube. The guy described a method that you disconnect the positive post of your battery. Then use a tester to see if the lamp lit when you attach the aligator head of the tester to postive post and the other end to positive cable you just disconnected fron positive battery post. If the light lit so there is a [/b]short and you check each fuse one by one to find the fuse that off the light.
Anyway I used this method on my truck. I disconnected all four positive battery posts. Then I saw the tester lamp light turned on when checked it between positive post and positive cable and light went off when negative cables that go to starter are detached from negative battery post.
Now the question is how is that possible?
I checked sll the cables that go from batteries to starter. Nothing unusual.