Mother in law

LM
My mil took our 8 month old son out to walk their dogs at 9pm at night without asking or telling us she was leaving. About 5-10 min later I go look for him in her room since it was his feeding time(we live with her) and they're nowhere to be found. I told my SO that I was not okay with her just doing that and he got really offended and said I don't trust his mother and idk what. It turned into a really big fight. Was I wrong?? Would you ladies be okay with someone taking your child out at that time of day without even telling you?? Cause I'm really confused as to why this turned into an argument.