The way I understand things, Is that Ketosis is all about keeping Carbs to a minimum (3xSS packs have 42.5g) and keeping the glucose supply in your blood near empty. Eating more protein/fat won't switch you back out . Based on Cambridge saying SS+ (60-70g carbs) still keeps you in ketosis, there's some margin for error here too (only two slices of breads worth though!)
If you have a high protein meal, like chicken and salad, all it'll do is reduce your weight-loss for that day because you won't have to burn quite so much of your own fat, it won't "kick you out" of ketosis.