When Banksys 'appear'... well, they don't just magic themselves there... does he do them at night? Stick a tent up against the wall for a bit? How?
Not sure about now he is rich and famous, but the method was definitely turn up at night with stencils (much quicker than freehand) some times in a hi viz vest (people ignore 'workmen' in hi viz vests) cans in a holdall. Spray it up nice and quick and walk off nonchalantly. It's possible he's added a workmans tent to his kit, would make sense if he was doing something that needed more time.
Doesn't surprise me it was covered up, the council didn't know what they were dealing with until after they'd 'cleaned' it. While some people know what a Banksy (and some other street artists) might look like, to a lot of people they are still just random graffiti and therefore vandalism.