@PawelK @hackaday @kravietz
The old ammo ham cans could contain a couple of microphones and a microcontroller. It could be mounted and left running triggering a speaker to give different audio indicators like the AIM-9 Sidewinder growl tones. The range would likely just be enough to fire on the vertical grid cell that it covers.

The microcontroller could be cheaper as it only needs to loop code until it detects the frequency and sound profile. The old MIPS based ESP32 had good enough DSP for this. The ammo ham can would help with the distance and for carrying. I would almost suggest a cell phone because of the microphone, cameras and beefy DSP but EMF Emissions get people killed. A STM Blue or Black Pill board would work and so would a Pico.

Parts could be salvaged from smartphones including a battery. Quality parts can be difficult to find but the microphones from smartphones should have good performance. It might be possible to physically disable the modems by removing the power and data lines but that might work on some 4G phones or older.