Sorry, can't reach the synth (tetra in my case, but should be pretty much the same) for a couple of weeks.
But to me, what you describe makes perfect sense (if your problem was only happening with sequenced patches, it would not make sense). Unless turned off, the oscillators are always producing a signal, even when no keys are pressed. You use the VCA as a gate to let that sound through.
(the following is in theory, since I can’t try it at the moment) When you start turning up VCA level, you are opening up the gate, hence letting the sound pass through. When you are not in unison 1-voice mode, you will start hearing sound from all the voices at once when you turn up VCA level. And indeed, this sound will not be related to the sequence, because the sequencer is a gated sequencer, it needs a keypress to be activated.
Turning up VCA level is something typically used for creating drones. But I never use it, and prefer to use a pedal to sustain notes, this gives you more control (and I would guess most people do it that way, and that will be the reason why your findings are not typically reported).
The reason I was asking what you like about your “droning” sequences, was to give you suggestions how to approach it in other ways (like adjusting your amp envelope parameters, or using the sequencer in “no gate mode” (I use that mode very often). A bit hard though with your description…