It's not that I assume just because she wasn't there the night that Mr. Banks walked by means she died, but the way the music sounded so dramatic and dreary, it seemed like it was trying to tell us it was too late to feed the birds.
It is a little somber, but being very late at night, she could have just gone wherever she slept. The close-up of Jane Darwell always gets me, knowing that she'd been around in movies for a long time, and the character's living conditions probably indicated that she wouldn't live much longer. Ms. Darwell did appear in one more movie after this, however, but her cameo in this is very profound.
"Great theater makes you smile. Outstanding theater may make you weep."