I have discovered the same issue in my UR44.
There is a signal on all inputs, although no cable connected and gain level is set to the left most position. The same occurs on the main outs btw, you can clearly hear (silently) music playing through loudspeakers, although the output gain is set to zero.
I had opened a ticket and was told this is a defect and I should send it for repair. Then I discovered this forum entry and asked Steinberg support if it’s a defect or not by referring to this forum entry. Unfortunately I have not received an answer.
I would like to understand if it’s a defect or not. I am attaching a screenshot from Cubase, which clearly shows the signal (EQ curve) - I have added pre gain for better demonstration.
