How did she know who the anons are when we are all anonymous.
Black box is an electric junction box.
Sticker has the London Transport / Transport for London (TFL) logo/roundel at top left corner.
It appears to be a TFL property/equipment ID registry tag.
Might be associated with traffic signals/pedestrian crossing.
Could be control box for matrix type traffic sign or bus stop info board.
Could it be a dead drop or meeting location?