Not 100% sure but I think it is something to do with being able to write to localstorage (e.g. if the site figures it can't write there then it is in Incognito)

Not 100% sure but I think it is something to do with being able to write to localstorage (e.g. if the site figures it can't write there then it is in Incognito). I read it was something like that.