Quadratt HF Audio Bleed?

Hi, first post…

Been using my 1U Quadratt as a simple mixer but noticed I’m getting some high frequency audio bleed/leakage from channels even when levels are down. E.g. audio sources going in to Channels A/B/C, and going out from D to audio out. All switches set to UNI and works exactly as intended, except for when all levels are set to zero I still hear some high frequency content of the various sequences coming through…

Just checked Quadratt manual and my setup seems to be exactly as described for audio mixing, so not sure what is going on. Am I misunderstanding module capabilities, or have I got a faulty unit?

Any thoughts appreciated…