Just because you see "POP alert" on a radar detector does not mean that you are truly detecting POP mode radar. Normal Ka radar can be alerted to as POP as well as other leaky detectors being detected which is very common. POP falsing from other detectors is probably why it was thought that POP was being used all over.

POP mode radar is an ultra-fast instant on radar mode that is only available on a few MPH units. It isn't a method of using the radar equipment, it is an actual mode that you have to enter on these units. POP mode is not to be used solely for issuing citations, only to preview a vehicle's speed. Then the officer is to switch over to normal radar mode to issue the citation. Because of this, POP is extremely rare, "winning the lottery" type of rare. In my experience, instant-on radar use is commonly referred to as POP, even by LEOs.

On our detectors I recommend disabing POP. This will eliminate annoying false POP alerts from leaky detectors and disabling POP will increase detection performance. Why does disabling POP increase performance? With POP on the detector has to stop going through it's normal scanning process and go back to look at the POP spectrum for a signal. Because POP is so fast (a 0.060 second burst) it has to do this a lot. All of this back and forth scanning decreases normal radar detection performance.

Importantly, and as others have stated, using multiple detectors at once will decrease their performance. Test them individually to judge their true performance.