I think I kind of understand what you mean, but I have a few but's...
The things that happened to your aerial photography are, quite frankly, your own mistake.
That has little to do with forgetting to watermark your images. You made a mistake with the conditions for selling your work to them.
If you made those images when you were actually employed by them, so not on freelance basis, then your work is their property.
The best thing you could have done, in retrospect, is have them sign a contract that you can use some of your work for your own CV/Reel.
Nonetheless, it seems you have learned from this experience anyway.
Secondly, when I google for my Terragen renders I find them on japanese/chinese sites, perhaps a russian site here and there and a dozen of small english websites who mention Terragen in some way.
I actually really don't mind they show my work there, it's actually good! If someone is interested to know and my name is not mentioned alongside the render then you can find my name very easily by reverse image searching using google. It's superfast and easy to do that.
I don't see a problem there, although it's not "chique" to post other people's work without crediting them.
On the other hand, since I'm an alpha tester for Planetside and a contributor to their website gallery I also have an increased "risk" of having my renders being associated with the product. That can't be a negative thing for me. Rather for Planetside

They sometimes "use" my work, but they don't earn money with it and if they would...how much would they make and how much would I make out of it? Zero.
There's no market for selling these kind of still renders.
In the past I had all my stuff on Zazzle, for instance, resulting in a whopping ZERO sales

So yes, I think you're a little bit paranoid, but given the history you described I definitely do sympathize with you.
However, as you said, with this render quality and resolution there's little rationale for watermarking it.
Cheers,
Martin