I also had a missed m/c at 12 weeks - I also had dark brown spotting which got heavier over a few days. I was also told not to worry because it was 'old blood' but i just knew something wasn't right.

Apparently from what I've read and been told, dark brown spotting is a classic symptom of a missed m/c and is usually the first indication that something's not right.

Good luck, I hope you have a scan showing everything is just fine in there!