One simple solution is to use duplicity (on which Déjà-dup sits) and produce a list of files in the backup:
duplicity list-current-files --no-encryption --time <timestring> target > list.txt
where "timestring" is given, according to man duplicity, as:
The acceptible time strings are intervals (like "3D64s"), w3-datetime
strings, like "2002-04-26T04:22:01-07:00" (strings like
"2002-04-26T04:22:01" are also acceptable - duplicity will use the
current time zone), or ordinary dates like 2/4/1997 or 2001-04-23
(various combinations are acceptable, but the month always precedes
and "target" is the path to the directory where your backup is.
In my case the full command was:
duplicity list-current-files --no-encryption --time 3D file://mybackup > list.txt
After this, you can open "list.txt" in any text editor and search for the filename.
If you want something more elaborated, have a look on https://askubuntu.com/a/486458/213084