Was Contractually Supposed To Pay Back Bonus, But Former Employer Never Asked Me To - How Long Would They Have To Collect?

I left my job 18 months in because of several issues - leadership blocking me from applying internally to new roles, offhanded sexist remarks from people on the team, my manager being inappropriate (no any one “major” event, just lots of micro-aggressions) I found another job, put in my two weeks, and left on good terms.

When I started the job, my contract stated I would have to pay back my signing bonus if I left within 2 years. No one mentioned anything when I left. In fact, I never personally even spoke to HR (when I asked about exit interview my manager said I could do one with him or just skip it). I am suspicious that they kept my departure as low intervention with HR as possible because I was going to a competitor but they really needed me to work those last two weeks due to some deadlines we had (and therefore didn’t want me walked - I should have been given the info I had access to), and possibly because they knew it was a bad look that the only woman on the team was leaving? That is just speculative, however.

It’s been 3 months. At what point am I “safe” from them attempting to collect my sign on bonus?