I wouldn't have thought that someone would have eaten half the bag of cookies, so I didn't bother to check carefully.
この場合、次のような言い方ができますよ。
ーI wouldn't have thought that someone would have eaten half the bag of cookies, so I didn't bother to check carefully.
「クッキーの袋の半分を誰かが食べてたなんて思いもしなかったから、注意深く確認しなかった」
I wouldn't have thought that ... で「…なんて思いもしなかった」
ーSomething like this would never happen in Japan.
「こんなこと日本じゃ絶対に起こらない」
ご参考まで！
I never thought that someone would have eaten half of the bag of cookies, so I didn't bother to check when I bought it.
こんにちは！
ご質問ありがとうございます。
先のアンカーさんが丁寧に回答されていますので、ここでは例文を挙げてみます。
A: Excuse me? The bag was open, and half of the cookies were gone.
Ａ：『すみません、袋開いてて半分クッキーなくなってます。。。』
B: Oh really? Why didn't you check it before purchasing?
Ｂ：『えっ？買う前になぜ確認しなかったの？』
A: I never thought that someone would have eaten half of the bag of cookies, so I didn't bother to check when I bought it. Something like this would never happen in Japan.
Ａ：『誰かが食べるなんて考えなかったから、買うとき確認なんてしなかった。こんなこと日本では起こらないよ。。。』
参考になれば幸いです。
カナダ 
日本 
