The Bad Wolf was a warning that could be found sporadically, but it was also notable to be a sudden coincidence that happened to occur all around every new stop the Doctor made with Rose Tyler.
A kid suddenly decides to graffiti Bad Wolf on the side of the Tardis (the Doctor's ship).
A maid in the 19th century describes that Rose has seen "the big Bad Wolf".
A news network from the future is called Bad Wolf TV.
A bay in Norway is called "Dårlig Ulv Stranden", closely translated as Bad Wolf Bay.
A special forces helicopter arrives. It's codename: Bad Wolf 1.
A bomb dropped during the London blitzes in WWII, painted with "Schlechter Wolf", a proximity to Bad Wolf.
A poster in a flying car, Bad Wolf in japanese characters.
The virus that destroyed files about Rose Tyler on the Torchwood's computer had the name Bad Wolf.
The play in an abandonen theater in another planet, Bad Wolf.
An alien commenting on military studies, citing the "classical Bad Wolf scenario".
In Cardiff, a nuclear power plant is called Blaidd Drwg, which is Welsh for "Bad Wolf".
"Bad Wolf" is an idea. A hint. A clue that the Doctor finds, but in reality, is a trail that points the direction that the Doctor is going. Bad Wolf needs to occur. Bad Wolf is always following them. Not the opposite.
It's so significant. And it works, because Bad Wolf was not to be avoided, it was supposed to happen.