I'm not sure if this applies in this case, but I remember Arthur Naiman in an early edition of the Macintosh Bible saying that Mac II class machines would act as though there was a second monitor attached as long as there was a card in a slot.

Initially I thought (without having any actual *basis* for my suspicion) that NuBus video cards are supported at a lower level than an SE cards, perhaps therefore meaning that an SE card wouldn't turn on without the system being specifically told by driver software that it was there, but I remembered that my SE/30 recognises its video card without any software at all. My bet is that yes, it still does even when a monitor isn't connected.