If i remember correctly they wheren't unknown but turned out to be soil bacteria that most likely (ccording to the researcher) contaminated the samples before flight. However, these contamination apparently survived 175 degrees Celsius reintake heat.
That is quite a lot, here at the university laboratory where i am currently in an internship we sterilise at 160.
According to most textbooks, that is sufficient to kill all bacteria. Guess not.
Nope:
"A fresh examination of the Viking Pyrolytic Release Experiment (PR) data26 supplies direct support for the formation and persistence of organic matter on Mars."