Dang don't think I'll be doing much with that but it was used to see if people was in the building
but they broke the rest of it that's all i could save LOL
Yes I think that the Gunn diode is the device in the centre of the waveguide cavity, but whether the resistor is there for static protection or just a leftover from where the thing was used I could not comment.
Many burglar alarm PIR's also have radar modules now, plus the normal PIR type sensors - in order to avoid false triggering they require both sensors to be triggered. So if you wanted to play with a radar module, that would be a cheap way - and presumably licence free?.