Very slight. Recall that liquid crystal displays (as found in a $1.99 multimeter from HF) also require AC excitation.
I designed stuff like that in those days. Signal processessing to reject false alarms was indeed the trick before microprocessors became common. It's still a good trick, but a lot easier to implement now.
Another approach was seismic. The sensors were geophones developed for oilfield exploration, basically a dynamic mike with a slug instead of a diaphragm. We made devices that could sense a man walking from a distance of several meters, a vehicle from several hundred meters or a chopper overhead, and it could tell the difference. It used a very small number of CMOS logic chips (less than half a dozen) to do it.