Alert if sensors haven't been triggered after a time?

I have various RF open/close sensors dotted about, and they run on batteries.

Some have a low battery signal but I don’t trust it (plus I can’t figure out what it is)

They are setup as MQTT sensors

Sonoff RF Bridge - Node Red - MQTT - Hassio

I’m looking for a way to get a notification sent if the sensor hasn’t been triggered after a week or two, which will prompt me to check the battery.

preferably via node red.

I can’t seem to work it out

I use the following with my PIRs:
on each triggering event I store datetime in an appropriate input_datetime.
then I have an automation that runs every hour and reports every PIR whose input_datetime is older than a maximum time set by input_number.
No Node Red here as it’s pretty simple.

Let me know if you need further details.

maybe you can try create a sensor using history_stats platform, use type: count