Hi Miguel,
I suspect that your advice above, asking, " Is it possible that you have some DC going through the signal chain that you are not aware of?", may hold the key to a solution. After duplicating Danny's setup, I found that inserting a DC Blocking Filter (under Miscellaneous) into the signal flow drastically reduces the background readings on the Level Displays.
Thus Danny should definitely try this. After having DC-caused errors upset a voice-detecting application some years ago, I routinely include a DC Blocking Filter in any design that could use it. It appears that the DC offset originates with the ADCs, from leaky coupling caps or whatever. It can easily be amplified by following blocks (such as a compressor set to bring up low levels). It's easier to get rid of it in software than attempt to fix it in hardware.
Best regards,
Bob