Unaware is an adjective. Unawares is an adverb. Unawares may sound like a colloquial variant of unaware, but in fact the word goes back centuries, and every dictionary lists it as an adverb. The -s here is one of the few remnants of a practice in Old and Middle English of creating adverbs with the -s ending. The suffix also lives on in words like towards and afterwards.
Unawares can mean without awareness, but it’s usually used to mean, more specifically, by surprise or unexpectedly. It appears most often in the phrase caught unawares.
But the personal equivalent of an earthquake followed by a nuclear catastrophe always catches us unawares. [Chicago Tribune]
More than 300,000 British savers were caught unawares when Icelandic internet bank Icesave collapsed in 2008. [Telegraph]
He noted that Russia, like the rest of the world, was caught unawares by the upheaval. [Wall Street Journal]