[jiggawatts]

Then detect the interference.

It's a similar problem to "jamming" in military radar systems. You might be half-blind while jammed, but you're aware that you are being jammed in the same way that you won't miss someone shining a torch in your face.

Multiple cameras across a wide range of wavelengths (UV-Visible-IR) would be fantastically difficult to hide from, for a range of parameters such as UFO size, altitude, speed, etc... To the point where it starts to become physically impossible, irrespective of technology level.

For example, various militaries have looked into tracking high-altitude spy planes via the disturbance they make in the air. You can camouflage the plane all you want, but you can't stop air from existing.