I'm seeing similar artifacts with my setup when I turn the power supply off--there are still spikes on the scope with no input power, just the IGBTs opening and closing. This has to mean there are dielectric fields present that are being messed with; these fields continually try to equalize themselves. With no current present, they don't do much, but they are certainly registering on the scope. Open circuit systems like these are just a bit difficult to explain away as noise. As you can see in my first scopeshot above, there are moments when both switches are open. During that time, something is finding its way into the coils and/or capacitors.
interrupt the cable to the circuit or switch off power supply (shut down)?
I ask that because after shutting down buffer capacitors are already charged and it takes 5 seconds for voltage to break down.