if (it|this) (was|were) a snake, it would have bitten you

said to mean that what a person is searching for is right where they are, but they have failed to notice it

The origin of the idiom "if it was a snake, it would have bitten you" is not well-documented, but it is believed to be of American origin. The imagery of a snake biting someone serves as a metaphor for the idea that someone has failed to notice or find something that is very obvious and nearby.

Come on, your watch is right here, Liz!If it was a snake, it would've bit you!
You've been searching for your phone, but it's right there on the coffee table.If it was a snake, it would have bitten you!
The answer to that question is in the first paragraph of the document.If it was a snake, it would have bitten you by now!
The solution to that problem was right under your nose.If it was a snake, it would have bitten you a long time ago.
You're frantically searching for your keys, but they were on the kitchen counter the whole time.Your friend chuckles and says, "If it were a snake, it would have bitten you!"