|Little Children (2006) [May McGorvey]: Dies (off-screen) following a heart attack after a confrontation with Noah Emmerich. We last see Phyllis in her hospital bed; we learn of her death afterwards when a nurse informs her son (Jackie Earle Haley). (Thanks to Tommy)
The Curious Case of Benjamin Button (2008) [Grandma Fuller]: Dies (off-screen) of old age/natural causes; we learn of her death afterwards when Cate Blanchett informs Brad Pitt.
The Big C: Everything That Rises Must Converge (2010) [Marlene]: Commits suicide (off-screen) by shooting herself. (Thanks to Alex)