Example where an empty folder, named local, is left behind. It seems like for each database there is a temp directory created with the same name.
corymintz: /tmp# mkdir a corymintz: /tmp# mongod --fork --dbpath a/ --logpath mongod.log --nojournal --port 30000 about to fork child process, waiting until server is ready for connections. forked process: 15362 all output going to: /private/tmp/mongod.log child process started successfully, parent exiting corymintz: /tmp# kill 15362 corymintz: /tmp# mongod --repair --dbpath a/ --logpath repair.log --nojournal all output going to: /private/tmp/repair.log corymintz: /tmp# ls -alh a/ total 163840 drwxr-xr-x 6 corymintz wheel 204B Dec 20 09:09 . drwxrwxrwt 16 root wheel 544B Dec 20 09:09 .. drwxr-xr-x 2 corymintz wheel 68B Dec 20 09:09 local -rw------- 1 corymintz wheel 64M Dec 20 09:09 local.0 -rw------- 1 corymintz wheel 16M Dec 20 09:09 local.ns -rwxr-xr-x 1 corymintz wheel 0B Dec 20 09:09 mongod.lock corymintz: /tmp#
If the directoryperdb options is used, no artifacts are left.