At the risk of sounding too glib or reductio ad absurdum:
The best code is impossible to spot, because it's not even there.
Put another way, a truism I've often heard from the most senior programmers (especially in the context of critiques of LOC as a measure of productivity), is that their best work is often writing less code, or outright deleting existing code. It stands to reason that this kind of work is much more difficult to spot if all one looks at is the codebase in its current state.
