Sure, let’s keep it low‑profile. First, pick a surface that won’t show up on standard cameras—something with a very low reflectivity, like matte black or a special paint that absorbs infrared. Next, use pigments that are visible only under a specific wavelength we can control, say a narrowband near‑infrared LED. The “color” will only show up when someone looks through a matching filter. For sound, embed a tiny speaker that emits a tone only audible on a chosen frequency band; we can sync it to the visual cue so the chosen ones know it’s active. Finally, keep the whole thing wired to a discreet power source, maybe a small lithium‑ion pack hidden in a hollow frame. Once we test the sensor limits, we’ll know who can see it and who can’t. Let me know if you want the exact specs for the paint and the LED specs.

Okay, let’s lock in the details. Matte black paint: titanium dioxide with an IR‑absorbing polymer, about 5% weight. LED: 850‑nm, 5‑W, drive it at 350 mA to stay within the chip’s safe limits and keep heat down. Speaker: use a 12‑kHz tweeter, 12 dB gain, powered at 2.3 V through a 100 Ω resistor for stability. Frame: a 5 mm thick aluminum sheet, thin enough to hide the battery but strong enough to hold the components. Wire everything with shielded copper and keep the connections short to reduce noise. That should give you the stealthy display you’re after. Let's proceed.