Fristly,
I'm not convinced that all Banksy's are actually banksys.
Though I've seen enough of them as I used to live in Bristol. my favourite is probably the one with the teddy bear throwing a petrol bomb, (mild mild west) mostly because this is one that I randomly saw just around the corner from where I lived in St Pauls, and actually laughed out loud in the street.
(the title mild mild west is probably a reference to the area of the country mostly described as the west country...)
Other than that this idea that banksy only comes out at night and hides his identity is probably rubbish, a lot of his work is in difficult to reach places, (like the girl hanging out of a window on a building in Clifton. I call BS on the idea that he shows up on his own, with multiple cans of paint in different colours, huge stencils and a ladder to actually reach that part of the building.
Most likely the dude works either with a person or with a group of people.
I saw some people creating something that was akin to the work that banksy does on a new building near to the square in St Pauls, a couple of people, with stencils and paint. -probably not actually banksy, but neither you nor I could probably tell the difference anyway as it's not like some old master where you can identify an artist by the brush strokes.
don't get me wrong. I do rather like the "work" that he does, but then I find a lot of graffiti (-especially when done well or with a joke or clever reference) quite good / artistic.