Turns out the PIR sensor (
https://lowpowerlab.com/forum/index.php/topic,1478.0.html) was an excellent test case for this problem: triggering the 433mhz outlet requires a fairly long OOK signal and that triggers RSSI detection in ListenMode nodes.
I've implemented the workaround discussed here. I now map the RSSI interrupt to DIO0 and select RF_LISTEN1_END_10 which causes listen mode to stop when RSSI is detected. In the interrupt handler I wait 8ms for PAYLOADREADY. And finally I restart listen mode.
Obviously doing this in the interrupt handler is far from ideal. But I really don't want the radio hanging in RX while some sensor library is spinning in delay().
This is now robust against noise. And I can measure the number of false RSSI detections.
One drawback is the number of IRQ's fired. For every listen period I get two interrupts when no signal is detected. And I even get two with RF_IRQFLAGS1_RSSI set if there is a signal. Luckily it's possible to determine the one that counts: RF_IRQFLAGS1_RSSI needs to be set and REG_OPMODE needs to be RF_OPMODE_RECEIVER.
The frequent interrupts make it a bit challenging to manage interrupt handling. Where one might have used LowPower.powerDown( SLEEP_4S, ..., ...) previously that code now always returns when the listen window hits. But I have it sorted out now.
I'll post some code in a few days if this keeps working.
Joe