Mysterious mail messages

[Nyantastic]

Each time I log in as root I am told that I have "x mail messages". However, these mail messages seem to be invisible:

$ su     
Password:
You have 7 mail messages.
root@orange:/home/nyan # mailx -u root
No mail for root
root@orange:/home/nyan # mailx 
Mail version 8.1 6/6/93.  Type ? for help.
"/var/mail/nyan": 0 messages
&

There is also no "mbox" file present in /root/ or in /home/nyan/, and the directory /var/mail is completely empty:

cd /var/mail
[nyan@orange] {01:15 11} mail 503 $ ls -l
total 4
-rw-------  1 _tss        _tss        0 May 24 09:52 _tss
-rw-------  1 avahi       avahi       0 May 24 09:52 avahi
-rw-------  1 nyan         nyan         0 May 24 12:49 nyan
-rw-------  1 cups        cups        0 May 24 09:52 cups
-rw-------  1 git_daemon  git_daemon  0 Jun 19 22:27 git_daemon
-rw-------  1 messagebus  messagebus  0 May 24 09:52 messagebus
-rw-------  1 polkitd     polkitd     0 May 24 09:52 polkitd

This is on a new system running FreeBSD 11. I have been using FreeBSD since version 7 and have never encountered the above message before.

What is more, the mail for "root" is being delivered correctly, and for example today I got 12 messages, when the above message said there were 8.

A google search leads only to the following old thread:

https://forums.freebsd.org/threads/39356/

 

[SirDice]

Are you forwarding root's mail somewhere else (/etc/aliases)? Not sure if that's the cause though.

 

[Nyantastic]

Are you forwarding root's mail somewhere else (/etc/aliases)? Not sure if that's the cause though.

I have not altered /etc/aliases from the default version. Root is receiving mail correctly from the daily security and other jobs. Even when all of root's mail is deleted, the message persists.