Banksy's Self-Destructing Painting

On 5 October 2018, famous anonymous street artist Banksy's painting "Girl with Balloon" was auctioned for £1,042,000 by Sotheby's London, immediately after which it partially shredded itself via a contraption hidden in the frame.
https://en.wikipedia.org/wiki/Love_Is_in_the_Bin

Soetheby's and Banksy are claiming that the auction house didn't know this would happen, and people seem to be accepting this at face value, but it doesn't make sense to me.

• This art piece has been in their possession for 12 YEARS. It's not like they just received it yesterday and stocked it on the shelf.
• Ok the frame had lights that needed power - no concerns about batteries lasting that long here. It's also easy to conceal the paper exit.
• "Yep, we'll just take this huge bulky frame and hang it in a small room next to extremely wealthy people without any scrutiny whatsoever, not even, you know, A BASIC X-RAY that will reveal the BLADES, MOTOR and WIRING hidden inside"
• Awfully fortunate publicity for Sotheby's, who now has an article on it on their website https://www.sothebys.com/en/articles/latest-banksy-artwork-love-is-in-the-bin-created-live-at-auction