Listening posts or observation posts.
In ww2 these would be logical if you are in a fixed position open to enemy ground attack like our Airfeild\GV Base\Port objects in the game.
Create a new object manned with AI troops. Salt 3-12 of them strategicly across the approches from the spawns. Enemy GV passes withing (x)yards the post phones in to the local tower. Only in that tower can you access a status display. From the main game map you see the usual feild or town flashing.
The post will only display that a vehical has passed it in say the last 2-3 minutes. Let the timer on the display change color as it ages. When a GV intialy passes, the display for that location will flash for say 15 seconds then settle to a solid color and age. If more than one vehical passes in short intervals like a raid, it keeps flashing untill the last passed out of range and starts the age timer. IF....a vehical stops in range and dosen't kill the post. AND the vehical sit's there past the age timer. THEN the post won't phone home until the vehical starts moving again starting a new timer.
Leave out what direction the GV was traveling. Might be fun to send jeeps off to find all of the posts and get them flashing to confuse the tower or just destroy them.
If you are in the tower you can access the current status of all listening posts. The posts are destroyable objects like auto ack with a similar rebuild time and response to base resupply. If destroyed, in the tower you will get an indication of this. Now the ord porkers have something else to pork to confuse the tower lurkers on the status of a GV assault.
I can see it now.
NOE GV assult on a town rushes in to kill eveything. Something big keeps taking out all the T34 and wirbles from the town. Hey George how did they see us?? I thought this was an NOE raid. They?? NOE??? You mean that TigerII killing us???? Yeah there Tiger, not werTiger.....Hey!!, did you remember to kill the listening posts?? I thought you killed them....Douh!!!!!

All this aside. We have sound, Storch, IL2, Ju87-G2 to help us find GV. Why not ask for an observation tower high enough to use binoculars with zoom like the 88's to look all the way to the spawns? If I were the base commander I'd at least have one guy with binoculars stationed up in the radar tower knowing I could be over run by GV at any time.