Similar to how people will read a temporary hand written sign on a door, but filter out a (newly placed) professionally made sign, because the latter looks too permanent.

If something has changed, using something out of place or temporary in appearance seems to be the most effective way of getting human attention... A bale of straw feels like it fits the bill.

Absolutely. A straw bale hanging from a bridge over a river is an anachronism. It doesn't normally belong there, and it's more likely to be noticed than a literal notice.