Time to try the Ultrasonic distance sensor with the panning servo mount.
The Ultrasonic detector uses the common HC-SR04 module. It emits a stream of eight 40 kHz pulses and provides an ECHO pulse whose width is proportional to the reflection time.
The detector uses a single servo that provides panning to allow detection in different directions. It uses an SG-90 micro servo which has 180 degrees of rotation.
The range is supposed to be 2cm to 400cm. That's certainly more than I'll need.
Here's a short range test (limited by the clutter on my desk).
A quick panning test to allow for detection Left, Forward, and Right.
I don't think that I'll be able to use both this sensor and a camera because there isn't enough physical space, but that's something to think about. Also, I would probably share the panning - but would I want the distance sensor to tilt with the camera?