After power up, it takes 30 seconds for the PIR sensor to stabilize. So during startup, the sensor will not work well. Try wait a while before doing your tests.
The Panasonic modules have very low output current (FET source) 100uA and needing a pulldown resistor. It's not enough current to drive an LED without adding a transistor. Drawing more than 100uA can cause false triggers.
The sensors also need clean power, so your scope ground might be affecting electrical noise if the sensor has a weird power supply. Or the probe capacitance is affecting your output.
If you want more help, post your schematic.