15. (3 points) Xiao Lan added consecutive even numbers $2, 4, 6, 8, 10, 12, 14, 16, \cdots$ one by one, and got the result 2012. Upon checking, she found that she had missed adding one number. What is the number that was missed? $\qquad$ .
【Analysis】Consider this sequence of even numbers as an arithmetic sequence with the first term being 2 and the common difference being 2. Let the total number of terms be $n$. According to the general term formula of an arithmetic sequence: $a_{n}=a_{1}+(n-1) d$ and the sum formula of the first $n$ terms: $S n=n\left(a_{1}+a_{n}\right) \div 2$, find the value of $n$, and then solve the problem.

【Solution】Solution: Let the total number of terms be $n$, then the last term can be expressed as:
$$
a_{n}=a_{1}+(n-1) d=2+(n-1) \times 2=2+2 n-2=2 n ;
$$

The total sum of these numbers is:
$$
\begin{array}{l}
S n=n\left(a_{1}+a_{n}\right) \div 2, \\
=n(2+2 n) \div 2 \\
=n+n^{2} \\
=n(n+1)
\end{array}
$$

By substituting values, we can find:
When $n=44$, the sum of the sequence $=1980$,
When $n=45$, the sum of the sequence $=2070$,
Thus: $1980<2012<2070$,
So the sequence has a total of 45 terms;
$$
\begin{array}{l}
45 \times(45+1)-2012, \\
=2070-2012, \\
=58 ;
\end{array}
$$

Answer: The number that was missed is 58.
The answer is: 58.