If it were a purge it would have mentioned it in the event log. It doesn’t sound like this player has been on long enough to warrant a purge.
Oh, I wasn’t aware of this. I mainly play single player offline (but I do have a few characters with a single base on official PVE servers though). Thanks for clarifying kevbal 
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Yeah, I know this is frustrating and I ran into the same problems when I first started out. My smaller bases would disappear when I would log back in and I didn’t know why. I don’t know what criteria the game uses to figure out how large to make the decay timer (like if there is a specific minimum number of foundations/walls/etc needed to max out the timer—maybe the game gets really generous when you have more than 20 foundations. I don’t even know if placable decorations count towards this timer). Is it possible that you made some edits to your base (deleted foundations/walls/ceilings/etc) which shortened the decay timer from the last time you played? You mention that you had 15 foundations but no mention of the number of walls, ceilings, etc. For example, if you only had one door frame and 5 walls and two ceiling pieces, you may not have had enough required content for the game to give you enough time for you to log back in. I will say that it’s hard for us to know the complexity of your build as there are many variables known only to the devs that we players don’t have specific knowledge of.
Personally, I see two ways of handling this going forward.
First, if you really must know what had happened, then this approach would help us to see if you really have a bug/glitch/issue or if your base was just too small. Recreate your house and write down the duration of the building items you place. So put down the 15 foundations and pull out the repair hammer and write down the duration of the decay timer. Place your walls, then record the decay timer. Continue doing this until you have recreated your home and then we can see if the amount of time is reasonable or not. Writing down each step will give us a rough idea of what the different components add to the pool of minutes in the decay timer. You can also give us what the time is for just the finished home, but more info is better…
The second way is to do what I did when my PVE-C base decayed about a month ago. I woke up on a floating double bed with two floating chests levitating next to it. At first, I thought the base was rendering in as a neighbor had built a huge base near me and was filling up the noob river with foundations. After a few minutes, I realized I lost almost everything so I had to start over. I figured out what size I wanted my new house to be, placed the foundations and put up the walls. Then I added foundations all around my new house and made sure they were snapped together to make a large footprint for my base. The important thing to do is to keep checking the timer with the repair hammer and to keep adding foundations until you max out the timer (168 hours). The foundations don’t have to be permanent. If you add other buildable components to your structure, you can delete the extra foundations that you don’t like. Just make sure that you don’t decrease the timer when you remove them. Since I don’t play online that much, I just log in once a week to reset the timer and then I play offline.
Lastly, if you can’t stand the thought of losing your creations, then either single player offline or renting a server may be a better bet for you. Even if you max out your timer, have the largest base on the server, and were there the longest, if you or any clan mates don’t log in time, everything will be erased. At least with single player offline, the decay timer is off by default and your creations will last (at least until your computer/console/hard drive dies).
I know this mechanic can seem annoying but it is necessary as it keeps people from pillar spamming and it cleans up abandoned bases all over the map. Too many structures create extra load on the servers which can make gameplay unenjoyable (freezing, stuttering, rubber-banding—the game does have these issues from time-to-time but they would happen more frequently if the devs didn’t have something like the decay timer in the game to clean up things.
TLDR: Your base was probably too small for the game to give you a reasonable decay time. Maxing out the decay timer (168 hours) would be the easiest solution to save your new base as long as you remember to log in before the decay timer expires.
Hope this helps you out. It is a fun game but it does overwhelm new players with tons of information at first.