I figure this is a parody of all those “Caution: Pickpockets [or sometimes Thieves] Operate in This Area” signs you see all over London, especially in the tourist areas. Or else someone is angry that their street art is being photographed, which seems a bit unreasonable. (In Shoreditch.)
Those signs are a boon to pickpockets. Because a good fraction of people, when they see them, will touch the pocket where they keep the thing they consider most valuable…