Placing a redstone dust under a detector rail makes it powered without minecart

Well, I did expect that the redstone wouldn't turn on when I placed under a detector rail. There are no other power sources around, I've checked.

What actually happened was that the redstone did get activated.

Steps to Reproduce:

1. Place down a detector rail, in any direction.
2. Dig a hole under it.
3. Place a redstone dust there.