After having this "Rosa" system be asleep for a number of hours, I stumbled upon the "locked" screen had come on (showing the current time).

There had been no accidental movement of the mouse that would have awoken the system. There was no noticeable loss of power. (The equipement is not yet on a battery backup.)

What would there be that would have brought the system out of idle?

Is there a System Event Viewer (being familiar with Windows' Event Viewer that would have shown events from a reboot)?

(As an aside, and probably only extremely coincidental, I also noticed that only one of several satellite receiver boxes had also rebooted, but that piece of equipment coincidentally happens to be on a battery backup. No other equipment shows signs of having been rebooted.)

Anything listed here?

Twenty-one lines, nine of which have *enabled in the line, none of which (of course) I would know what the data means.

(I suppose I should make a sig that says, "I am far worse than an untrained soldier.")

Well, why not post the output here? ....we may be able to assist further. Please enclose output within code tags.

Hi...

I'm not sure but if you don't want to see this behavior, you can go into your system's BIOS and disable the "Wake on LAN" feature. There is also a method using the OS mentioned here. As it's written with Ubuntu in mind, you might also have to install gedit. Hopefully, one of those two methods will take care of the problem.

Regards...

I was hoping this wouldn't come so soon..

I have a lot of traffic on the LAN, none of it is the "magic packet" that would bring a system out of idle.

This "Rosa" is (was) a two day old installation, so whatever would be as standard installed feature/function of Linux (kernal or otherwise) would be the only things listed in that "wakeup" file. So, I was, perhaps, in error believing a respondent would already know what would be in this file, thus not including the file contents.

N.B., the only changes I have made to it is to run the "upgrade/update" (which did pull down a lot of stuff), install NoScript and AdBlock into Firefox, and install Hiawatha, MariaDB, and PHP7 (but have not configured any of it - but they are running).

My experience with running the same programs on Windows XP and three installations of Windows 7 and Server 2003 and 2008 has prompted this question - because I know nothing about Linux or its behaviors.

From "wakeup":
*enabled:
GBE, OHC1, EHC1, OHC2, EHC2, OHC4, XHC0, XHC1, PWRB
*disabled:
PB7, SBA2, PS2K, PS2M, ECIR, POPC, OHC3, EHC3, PE20, PE21, PE22, PE23

You said another device in the house rebooted as well, that makes it sound like an isolated brown out that only affected part of the house. You said the other device was on a UPS, but that UPS could be malfunctioning.

Is the BIOS on this computer set to turn on when power is applied? Usually the options are "Stay off", "Last state", or "Turn on".

BIOS is set to "Stay Off" when power returns.

A malfunctioning UPS would have caused all sorts of mayhem on the other equipment plugged into it.

And if it was a brownout which had caused the system to reboot (a reboot has NOT been confirmed), I ask again, is there an Event Viewer that would log a bootup sequence?

(Making a SWAG on the sat rcvr, maybe a firmware update that forced a reboot?)

I'm not too familiar with the logging behavior of Mint, but most Linux systems have a /var/log/messages file with time stamped logging info from boot.

In /var/log/ there is syslog, and the contents do not indicate a reboot. Therefore, what I saw must have been something taking the system out of standby/idle.

Where do I determine what "power savings" configuration the system is currently using?

Found the Power Management: set to turn off the screen after 10 minutes, and never suspend.