Covert Proximity Sensing and Alternatives

Projects General Guidance
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Hi, I'm still a bit new to arduino and the physical computing world, but I have been able so far to get my ideas to work. I do have one issue that comes up a lot. I do a lot of interactive work using sensor, proximity sensors being my favorite, but I haven't figured out how to use a sensor without it being obvious that there's a sensor.

I want a single person's proximity to an object to create events, but I don't want them to know how they are triggering the event.
My current piece involves using a proximity sensor and a person's proximity to a hole in the wall to cause a plastic spike to slowly emerge and retract from the wall. (I attached a photo of the project and the sensor location.) Right now, when people approach the hole they can see my sensor, so it kills the magic rather quick.

This is pretty much the sensor I'm using. It may not be the exact model, but it's close enough and is the same size. Ultrasonic Range Finder - LV-MaxSonar-EZ1 - SEN-00639 - SparkFun Electronics

Any suggestions for effective ways to hide a proximity sensor without messing up its values? Are there any smaller prox sensors out there that might work better?

I'm also wondering what alternatives there might be to using a proximity sensor that might be easier to conceal?

Thanks in advance!

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If the "wall" is made of something like wallboard/gypsum board then it has a paper skin on both sides. Perhaps you can make a hole for the sensor but leave the paper coating intact. I don't know how badly that will degrade the ultrasonic signal but it's worth a try.