From the original BBC news article: "One possibility the scientists had to rule out was that the ammonia came from the air bags of the failed Beagle 2 mission. Analysis revealed that the ammonia's distribution was not consistent with this explanation."
I don't know how they ruled this out, but assuming their work stands up, it is probably not a human source.

Since Beagle has been missing for over seven months, and given that ammonia is not likely to be stable, if it is from Beagle- or any other human source- either the signal is spectacularly weak (possible) or Beagle released very large amounts of ammonia. I guess we have to wait.