I found some youtube videos playing this GLaDOS sound file which I don’t recall in the game at all. All the videos are fan made, none show the quote being made in game.
I’ve beaten the game about 4 times but I’ve never been able to discern this from her final words.
Was this included in the real game or is it an extra sound clip?
GameFAQs has a script with all the lines from Portal “There really was a cake” is indeed in the script, GlaDOS says it when defeated.