Todd was quicker on the detective work than me.
The CCC has an “abbreviations” page towards the back with some of the main abbreviations used. This one does not appear there.
What I do is go to the Index of Citations in the back (presuming you have the big green Catechism that includes this—some of the smaller pocket editions of the CCC don’t have it). It is really super handy. It goes through all the books of Scripture, Church documents, and finally “Ecclesiastical Writers” (i.e. the saints). Go to “T” and you can find St. Thomas Aquinas and a list of every place he is cited in the Catechism.
Since your reference was from a footnote in paragraph 293, you just scan the right side until you find that paragraph number. In this case, it appears under the work In Libros sententiarum. From there, my next step is usually to Google the work to see if I can find the full text somewhere on the web.
It is a great book, but it actually doesn’t contain every footnote. And the reference in the OP is one of those that are not included.