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Obviously making stuff miniature will help make the instrument smaller, but there is a lot that goes into taking an accurate gravity reading to detect “small” things. I don't recall off the top of my head, but I think a 6-foot tall tunnel at 20 feet deep would be undetectable with even the best gravity meter and conditions.

The elevation needs to be known within a hundredth of a foot as well. And how do you get the data out of the device? I'm sure in the lab they have it wired up to a laptop or something. I suppose they have miniature data sending modules though as well - like bluetooth? I could see where you could “seed” the border with these chips, and see how their readings CHANGE over time.

They would also detect vibrations from foot and vehicle traffic as well if they are sensitive enough. Or - have them combined with a miniature seismic sensor.

I've read where they have, or are developing, sensors for storm watching that measure air pressure, wind speed and velocity and temperature and then relay that to the ground. The sensors are the size of a grain of rice! An airplane drops a bunch of them into the storm cell and away they go.

a 6-foot tall tunnel at 20 feet deep would be undetectable with even the best gravity meter and conditions...

You are correct. Seismic technology, which is highly refined thanks to our geophysics friends in the petroleum industry is far more do-able from a practical standpoint. Ground penetrating radar is also feasible within certain constraints.

"They would also detect vibrations from foot and vehicle traffic as well if they are sensitive enough. Or - have them combined with a miniature seismic sensor."

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I spent several years working on development of micromachined silicon microsensors -- including suspended-mass motion sensors like these.

Believe me, the sensors discussed in this article would be far more effective seismometers than they are gravity sensors. The problem is in nulling out seismic or other vibration signals in order to detect micro-gravity variations...

Perhaps one approach might be to encapsulate them in a controlled-viscosity damping fluid -- perhaps a non-Newtonian one... Even so, temperature effects on fluid viscosity might have to be controlled or compensated...