There are simple and cheap USB dongles that can detect 868 MHz devices. I got myself a "nanoCUL" for this purpose.

This consists of an Arduino Nano and a Texas Instruments CC1101 radio chip which can be connected to a PC via USB.

There are several firmware projects for the dongle. SignalDuino, culfw, a-culfw, ...

I tried them all but unfortunately the protocol is not supported. So I must dig deeper here.