The system is running the latest Tumbleweed snapshot and zypper version shows
"1.12.12".

Suppose that a package "xmms" is not yet installed, and a package called
"abcde" does not exist.

In interactive mode, attempting to both packages causes xmms to be eventually
installed:

# zypper in abcde xmms
...
...
Overall download size: 2.5 MiB. Already cached: 0 B. After the operation
additional 8.8 MiB will be used.
Continue? [y/n/? shows all options] (y):

However, in non-interactive mode, zypper does nothing:
# zypper --non-interactive in abcde xmms
...
...
No provider of 'abcde' found.

The behaviour of non-interactive install should be consistent with an
interactive install.