You badly misunderstand something. The Streisand effect is an unintended consequence whereby trying to minimise attention actually increases it. "Banksy" has known all along that maintaining the mystique of secrecy would create attention where otherwise there would be none. So the exact opposite of the Streisand effect. His secrecy is to create attention, not avoid it.

The Streisand effect in this case would be Mayor Bloomberg's reaction, threatening to destroy any of Banksy's artwork found and arrest him if possible.

Honestly, he couldn't pay for that kind of publicity, and unless the good Mayor is actually in on it, Banksy must be grinning from ear to ear at the irony.

I'm not talking Banksy's secrecy. I'm talking about the effect of provoking authorities and having their condemnation generate publicity that provides more incentive for Banksy.

/Ironic/ use of the streisand effect == the Banksy effect

There ya go...