Expression-oriented languages don't need explicit returns. However, your point is absolutely correct: there must be a constant overhead, even if it's just a single bit; otherwise there would be no way to tell what should be treated as a program and what should be treated as a literal string/value (even if it just-so-happens to be a valid program).

Since this overhead is constant, it becomes negligible as the sequence length increases. Yet we should take it into account, e.g. by saying the upper bound is 'N + O(1)'.