sshd regular exit code 255

I'm a novice at network things. I enabled remote-login on my home mac (OS 10.12.5) so I could git between home and office. To make it work, I had to tell the wifi to fix my Mac's IP and to redirect Port 22 to it. Works fine. My log file, however, has thousands of messages (one every few seconds) of the form:

com.apple.xpc.launchd[1] (com.openssh.sshd._hex_[_int_]): Service exited with abnormal code: 255

Nothing in the log explicitly mentions a refused authentication, and I don't seem to have an auth.log or secure.log file (other posts suggested to look in there).

Any idea what's going on here?

That's the log entry performed when sshd denies a connection. You're probably seeing unauthorised users trying to brute force their way in by trying common usernames and passwords. There used to also be a log entry directly from sshd itself telling you the IP address and username attempted, but Apple has decided to labotomize standard UNIX logging from sshd.