I would say it really depends on the circumstances.
I am one of the developers of Maxima, and I also routinely use Maxima in calculations employed in physics papers. Maple, too, and occasionally, Mathematica.
Computer algebra can be used for trivial things. By way of an admittedly trivial example, I may run into a trigonometric expression like
4*sin(x)^3-3 sin(x), which looks vaguely familiar, so without giving it a second thought, I'd feed it to Maxima,
radcan(trigreduce(4*sin(x)^3-3*sin(x))), and presto, I get
-sin(3*x) which goes into my paper. Would I mention that I used Maxima for this? No more than I'd mention that I used a calculator to calculate, say, 10/7 = 1.4286. Same goes for relatively straightforward integrals and other simplifications, even if the intermediate results are difficult for humans to keep track of.
The one potential issue with this is that computer algebra systems are programmed by humans (like yours truly) and may not be free of bugs. But that is kind of a user beware thing; it is best to check critical results by recalculating them in more ways than one, perhaps using more than one computer algebra system. In the end, it's no different from making a mistake when doing calculations on paper: always double check your critical results!
In short, in these situations I would not cite the use of computer algebra or expect, as a reviewer, an author to cite its use.
The situation is very different when the use of the computer algebra system is more elaborate. When it's not a single command line but a lengthy, elaborate program in that computer algebra system, designed to set up, solve, and simplify a problem. In that case, yes, I'd very much expect not so much a cited reference to Maple (or whichever CAS is used) but perhaps the actual computer algebra code included in the paper as an appendix with a brief explanation or, if it is too lengthy to be included, then deposited in a public archive and referenced. Otherwise there is no reproducibility: how do we even know that the program (perhaps written by a researcher with limited programming experience as his expertise lies elsewhere) is even correct?
From the wording of the question, however, I get the impression that it is a case of the first variety: the question itself suggests that once the reviewer recognized that the solution was likely obtained using Maple, he was able to obtain the same result with ease.
So no, the simple fact that a result was copied from Maple is not a reason to cite Maple, just as copying a result from the display of a calculator is not a reason to mention that calculator.