Maybe a grommet of some sort? My first guess would be your BCD, a pocket, drysuit, or something like that. My second guess would be random debris that got caught in your gear while diving.
Personally, I expect it's a low probability that's something important if you haven't spotted a major issue yet. But low probability is still not zero. Do a "function test" on all your gear. In other words, pressurize and breathe from your regulators. Inflate your BCD to max and check for leaks. Put on your BCD and make sure it fits and feels right. Do similar with your other gear (mask, fins, wetsuit, etc). It might feel silly putting on all that gear at home, but better to discover it now, than when you're at the dive-site.
Something that works for me, is I dive with a LOT of redundancy. For example, if I service a set of regulators, I don't have any reason to think I messed up, but I'm also not that worried because I dive with redundant air. I also patched up a BCD with a big hole a while back, and wasn't worried there because I had 3 other reliable ways (DSMB, finning, or "climbing" the shore) of returning to the surface if the BCD failed.