I think I figured out the problem with MRTG (Multi Router Traffic Grapher) and the missing log files [1].
You see, when I installed the operating system, one of the optional packages to install was MRTG. So I decided to install it. But unbeknownst to me (and totally unconfigured mind you) as part of the installation, it was already added to cron but not as part of the file you get when you run crontab -e (because you know, just by the act of installing the software you must want to use it immediately, even if it isn't fully configured yet, right?).
So every five minutes, two copies of the software were being run.
So no wonder the log files were getting munged.
Grrrrr.
You'd think that after ten years of Unix administration I'd get the hang of things, but nooooooo! I guess I still expect to have to configure packages and add them manually to cron if I want them.