No one in our world wide network has ever reported any life threatening accidents with home scale biogas. The tanks we work with can't explode if that is what you are thinking, because they are filled with water. Water and dung. They store very little gas, and it isn't under any particular pressure.
Solar CITIES demonstrated the safety of INDOOR biogas in an unventilated room at Mercy College for over a year.
See https://www.youtube.com/watch?v=-k1FCVwjylI
See https://www.youtube.com/watch?v=oDbXsYVL5NA
We actually started with a small biogas system in my upstairs bathroom in Germany, using my babies diaper waste as starter material (solved that problem! "Who wants to change the baby's diaper?"
"I do, I do"!).
The feedstock then became the uneaten baby food.
See https://www.youtube.com/watch?v=cVFHtLAv30g
See https://www.youtube.com/watch?v=kFOapalN5uA for the first flame test inside in 2010
Our three IBC system that we developed at the SEKEM farm in Egypt in 2010 was implemented indoors in a favela of Sao Paulo, Brazil, by Paulo Mellett in 2014.
See https://www.youtube.com/watch?v=1W6V3QGs9fA
Question:I mean REALLY safe?