Re: Getting FATAL: terminating connection due to administrator command

 > Is there any other way [than SystemTap] I can attack this issue
 > [figuring out who sent a SIGTERM]? Attach GDB?

gdb won't tell you who sent the signal.

I don't really see any specific reason why sigmon.stp wouldn't show a SIGTERM if
  one was sent.

An alternative could be to use auditd with something like this:
# auditctl -a entry,always -S kill -F a1=15

Regards,
Adrien Kunysz