Game Mode: both
An inactive detector rail (one that does not have a minecart on top of it) outputs a redstone signal through a block.
Screenshot 1: shows the bug.
Screenshot 2: shows that the rail powers through the block.
Screenshot 3: shows the bug again..
Screenshot 4: shows the bug and shows that the detector rail isn't supposed to power anything (no output)
Screenshot 5: shows that removing the block removes the power to the rail
Screenshot 6: when you move the cart of the detector, replace the block / powerrail, it gets powered again.
Screenshot 7: if you place a block (after screenshot 5 where you removed it) it doesn't power (this is the expected behavior)