To save others who (like me) didn’t know what an IKEA Vallhorn is…
- Wireless PIR motion sensor
- Zigbee
- 2x AAA rechargeable (much nicer than CR2032, or Sonoff’s CR2450)
- VALLHORN wireless motion sensor, smart/white - IKEA
My guess is like many other IKEA Zigbee products, IKEA have “extended” the protocol for their own ends (like local and direct pairing to a device), and whichever Zigbee stack / Add-On you are using will need a “quirks file” to make the device function like any other Zigbee sensor. There are also blueprints for some other IKEA devices.
Which Zigbee stack are you using, and have you checked if they have a quirks file (or a blueprint) for the Vallhorn?
(searching here shows deConz from 2024-01 which suggests others like ZHA might be at least working on it - check GitHub perhaps?).
Here’s a previous worked example fopr ZHA…
If this helps, 
this post!