Our medium sized girl Millie was a rescue pup and we have always wondered about her lineage. She is very quiet and calm, about 45 lbs. Appropriately, she has a ? on her side.
I love to play "guess the breed(s)". Mostly the owners enjoy chatting about their dogs and their possible heritages. Occasionally someone gets miffed. In that case, I apologize for offending them and move on.
FWIW, Millie looks like a beagle/terrier mix to me. Whatever mix she is, she's adorable.