Here's the situation: She was an undergrad, I was a grad TA. At that point, our interactions were totally professional and limited to the classroom. However, she and I both since graduated and left the college/city where we got our degree. I now am at another university working as a postdoc and she is working outside of academia. We ran into one another and started spending time with one another. Our relationship has been platonic, but I get the sense that it could go further, which I wouldn't necessarily be opposed to. I can't imagine that there would be legal problems with pursuing a romantic relationship, but would there be ethical or professional concerns (as an early career academic)?
There are professional/ethical rules at many universities relating to dating between students and staff, but these do not extend to imposing a permanent moratorium on dating between people who have previously attended that institution. Essentially you are asking if there are any professional or ethical issues in dating another adult who has no connection whatsoever to your current employer. I cannot see any issue.
All the relevant ethical guidelines I have ever heard of relate to romantic relationships with someone who is currently your student, professor, boss or subordinate. None of them would apply here. Nor do I think that the circumstances would lead anyone to think that there was a relationship while she was your student, particularly given the lapse in time.
I see no problem.
I have two colleagues who have since married ex-students. This happened several years after graduation and they are, well 3 out of 4, all working at the same institution...
No issues, even the employer is fine.
But the employer is VERY clear about non-appropriate contact / behavior with current students...
Let's forget specific rules and regulations for a moment and think about why dating students is forbidden. There are two main reasons:
The possibility/likelihood/perception that you will give preferential treatment to your partner over other students (e.g., higher grades, better access to facilities, etc.), even if this is done subconsciously.
The possibility that you are abusing your position of power over the student to coerce them into dating you.
Clearly neither of these applies to an ex-student. Doubly so given it was a few years ago. Triply so when you're now at a different institution and the student isn't even in academia any more: you're no longer in a position to grant favour and you no longer have power that you could abuse. The institution you first met at is no longer in a position to tell you what you can do, and the institution you're at has no reason to care about your relationship with somebody outside that institution.
If they were a recently ex-student, it's possible that people would think something had been going on while you were their TA but, at this level of removal, that doesn't seem like a credible allegation.
If she is outside of the Academia now, no one on the Earth can say anything. The whole ethical problematic of the romantic relations is rooting in that it may cause distrust in your impartiality, if you are grading her work. It is because it is not enough to be impartial, you should also seem impartial.
If such a relation happens, the most typical reaction of the community (and the co-workers) is, that they totally ignore it. It is not their business and they follow it. Furthermore, everybody wants to keep a distance from any hot topic, and it is hot.
It would be even likely so, if she would be inside the academia, but in a different institute or department. If she would be your co-worker or student, then you should likely follow some regulations, for example you wouldn't be allowed to grade she in any sense (you should ask a co-worker to do this instead you).
I see no problem with it, as long as your employer is fine, which I don't see why they wouldn't be. You're working in an entirely different city than where you worked/graduated, and now work for a different college that she's not affiliated with at all. I see no issue with that whatsoever.
If you want weird/moral/ethical dilemmas, try this on for size: my wife (happily married 7 years and counting) used to be my little sister's babysitter/chauffeur. She's 6 years older than me and lived down the street when we were younger.
Cue my parent's divorce, and getting us 3 kids to school was a challenge due to now only one parent, no one being old enough to drive, the buses not coming near us, etc. I was a freshman, and the high school was close enough to walk to, but the middle and elementary schools were further away, so my little sisters needed a babysitter/chauffeur
My mom hired the girl down the street (future wife) to drive my sisters to and through it all, we started hanging out as friends. Junior year came, and the weird talk about feelings came, etc, etc, etc. Because of this, she chose to quit taking care of my sisters for moral/ethical reasons (she wasn't fired; my parents didn't even know). I graduated in June, proposed in November, and we were married by May of the next year. Still going strong, though we weirded out my family a bit at first ;)
It depends when the relationship was initiated. If it was strictly professional when you were a TA and then this new friendship/potential romantic relationship was formed after it should be fine. It would be a different story if you were for example flirting or messaging when you were a TA as that would be viewed as the romantic relationship starting to develop when you were in a position of power and you could definitely get in trouble for that.