Thanks Macnugetz...
After deleting a monitor and it's parent group via SOAP, the monitor was only retrievable using search. It was not found in Orphaned Objects. Clicking on the monitor opens it as normal to view it's configuration details.
However I saw that if I disabled the monitor (click on disable button), it afterwards showed up in Offline Monitors.
In the end, instead of a simple deletion, I had to retrieve the configuration details of the monitor, then update the configuration to disable it (and adding a "deleted" tag for reference), then deleting it (that's 3 different SOAP calls for one monitor - a real PITA).
The monitors then are visible in Offline Monitors, but still cannot be deleted. It turns out that I have to stop/start the ipmonitor service, and then they can be deleted manually using the server web interface. Not very satisfactory as the server(s) are in the prod environment and the operations guys hate requests to restart services.
Regards
jp