Since it doesn't make sense to number every single equation...
I used to number only equations I referred to, but then someone pointed out the following. When you write a paper and make it public, you are de facto allowing other people to talk about your paper and simply because you in your paper see no reason to refer to some equation does not mean that other people reading your paper will not do so. Hence as a friendly gesture to your readers, you should number all your equations, and in this way allow them to refer to, say, equation (n) in your paper, instead of having to come up with some complicated reference. Since that time I have tried to number all my equations.
Nobody seems to have mentioned much about teaching--- perhaps because the original question itself makes no mention of teaching having anything to do with the desire to return to academia. This is a kind of elephant in the room.
I should admit: I'm on the academic side, I have not personally tried to make this kind of transition, and I have never been in a position to evaluate somebody making this kind of transition. But it seems to me that if you're reasonably current with your research area, and publishing papers, and meeting people (as suggested elsewhere), your biggest obstacle may be teaching.
Presumably you have no teaching experience over the last n years, and depending on your grad school experience, you may not have had much then (or it may have been a different sort from what professors do). This may matter. I don't know how to begin building a teaching history, while working a full-time job.
You may need to overcome the suspicion that will find teaching low-level service courses boring for the same reasons you find your current job in industry boring. Imagine the skeptic on the search committee who asks, rhetorically, "Who wouldn't be an academic if it were all just learning, writing papers, and talking to enthusiastic people with the same interests?"
Even with stellar references and a personal connection or three in the department, someone will ask: can you teach? Do you want to? What's the answer, and how do you convey it on your CV?
I don't have specific advice in this area, because it depends on where you want to work, and your own background. If it is possible to do pedagogical things in your current job, or service/outreach to non-specialists or students, perhaps that would help. Maybe actual teaching (on a per-course basis, not as tenure-track faculty) or volunteering would help. My feeling is that you need to do something to address these issues head-on, to confront both any genuine gaps in your CV, and the biases and prejudices you may face simply because you are changing careers.
Best Answer
First, I do it all the time and don't really see the objections. A phrase like "In [S] it was shown..." is a good alternative to "Siegel showed, [S], that ...".
Out of curiosity I did some cursory research and looked up the citation habits in Annals of Mathematics 1958. There one author (R.D. Anderson) does use "In [1] we considered..." and "an argument in [4]". On the other hand Dold sticks to phrases like "due to J.C. Moore [11]" or "defined by Eilenberg-MacLane [4]" as opposed to "defined by Eilenberg-MacLane in [4]".
So the habit is quite old. On the other hand, the list of references at the end of the paper has not been around forever. Again some research I did during some idle moments in Oberwolfach showed that a shift took place in the 1940s-50s. Before, references were handled either inline or via footnotes. Clearly these cannot be used in noun form. Instead,repeated references to the same paper were done using the rather clumsy phrase "loc. cit." or "the third paper by Siegel, cited above". Maybe the objections come from this time.
Edit: After reflecting on this issue a bit more I found that even though this style of citation is per se admissible it does have a pitfall which one should be aware of: It makes it all too easy to avoid naming an author explicitly. I consider it as very bad practice if somebody's name appears only in the references.