OK - now that's interesting. I made the mistake of assuming that since the video was streaming to the DVR just like a wired camera that the DVR was detecting changes in the stream and deciding according to it's own sensitivity and region rules whether or not there has been motion. It's not...
I was recently told that the XVR5108S is not "guaranteed" to work with "other brands" of IP cameras (whatever that means) but I found out that it's accurate.
The XVR5108 connects to a generic ONVIR IP camera just fine, shows the live video (preview) just fine and records (regular -green-...