"Oblivious" used to refer to forgetting, but is now often used for this sort of failure to notice. It lacks any special connection to warning signs, as do my other suggestions. Definition of oblivious from the Merriam-Webster website:
1 lacking remembrance, memory, or mindful attention
2 lacking active conscious knowledge or awareness —usually used with of or to
They were pushing and shouting and oblivious to anyone not in their group. —P. J. O'Rourke, Rolling Stone, 14 Nov. 1996
Prentice looked up from his food, which he had been steadily shovelling in, completely oblivious of everyone. —Antonya Nelson, New Yorker, 9 Nov. 1992
Oblivious of any previous decisions not to stand together … , the three stood in a tight group … —Doris Lessing, The Good Terrorist, 1985
Father was oblivious to the man's speculative notice of his wife. —E. L. Doctorow, Ragtime, 1974
She rested now, frankly and fairly, in the shelter of his arms, and both were oblivious to the gale that rushed past them in quicker and stronger blasts. —Jack London, Burning Daylight, 1910
And one I happened across and liked: "In my career I have never felt that my being a woman was an obstacle or an advantage. I guess I've been oblivious." -- Carole King
It seems to me that other close words, more casual and apparently less specific, include clueless and dense.
Again from Merriam-Webster website: Definition of clueless include "completely or hopelessly bewildered, unaware, ignorant, or foolish."
And as to dense, Merriam-Webster defines it as "slow to understand," making it the loosest fit for the word sought. But it seems to me that the word is used as I suggest, for a failure to see what is obviously in front of you.
But I did not find instances of careful writers using the words the words clueless or dense as I have suggested.