It's a pretty normal phrasing. Though, I suppose it more usually comes with some sort of time. "I'll be back in 5 minutes." Absolutely normal. "I'll be back.". In the falling tone that he uses in the movies, is a little dramatic (especially since it's all he says. Nothing before or after). Usually, if a person is saying JUST "I'll be back.", chances are they would be saying it reassuringly (with a higher pitch on the word back). In the movies he says it like a normal declarative sentence, which makes it slightly different than anything people would actually hear.

All in all, it's only like a 10% difference; it's mostly totally normal to say that.