There are two parts of "terrain" that have no real physical geography in the traditional sense but have significant military applications, the cyber realm and the electromagnetic realm. A lot of what goes on in these realms are classified and I won't get into very many details because frankly I'm just beginning to come to an understanding of what goes on in these areas.
First the electromagnetic spectrum is not man made, but it is what we use for wireless communication and RADAR. Everything we put out there in the EMS is some sort of signal, and whether that signal has been modulated to hold data (such as radio or data links) or not doesn't matter one whit when it comes to detecting that frequency. Some emissions aren't modulated to carry data at all, such as RADAR or radio beacons.
The only man made piece of military terrain out there is the cyber domain. What you are reading this blog post over right now is considered military terrain. The internet has been militarized.
Now, the EMS and Cyber terrains are not exactly "lethal" in that bullets, bombs, torpedoes, and missiles aren't arriving through them and killing someone at the other side. What EMS and Cyber do is largely help clarify and define the battlefield. A RADAR operator is using the EMS to clarify the physical battlefield, the enemy is using Al Jazeera to push out their information warfare messages, and some governments are slipping viruses to other governments.
The common denominator here is that this is all technology driven, and it is usually "high technology" as opposed to "low technology." I can't build a cell phone, I can't build a radar system. I can't even build a simple telephone. About the best that I can build myself with common parts is a telegraph key and crystal radio transceiver to send it out over the EMS. When I say I can't "build" I'm not saying I could assemble one from parts, I assemble computers from parts all the time. But that is not the same as building.
So how do you protect yourself in these realms? Hide with pride and keep your anti-virus up to date. If you are going to use high tech communication tools, you should use low tech encryption at a minimum. Things like platen codes or book codes for written communications, code words for verbal comms. If you need to give your location on a map an easy way is to have an agreed upon "point A" and "point B" and then give your azimuth to each. I am 47 degrees from point A and 270 degrees from Point B. Having a 3rd point is necessary if you will be traveling between point A and B. Of course speaking in code over a single channel radio is useless if the enemy has direction finding capabilities.
There are ways to defeat direction finding, but I won't go into them suffice to say that it is worth your time to look it up.