Hello Jean!
That's fantastic! Your dad's escape was succesful? Could you write more concerning his debriefing with general Collins?
If you'd live only 5 km away from the abbey you would also know or find out some details I guess 
Thanks to your hint I found another hint to the camp below the abbey, but only that there was the camp behind barbwire and always guarded:
www.peter-zenker.de/documents/Zwangsarbeit_SU_Langfassung_300dpi.pdf (only in german, concerning forced labor in Siegburg and therefore not so easy to find when lookin for POW). The camp was in the "Rosengarten" (rose garden), but it wasn't a rose garden until 1958. There were several camps for forced laborers in Siegburg, but it seems as if some of them were also used as POW camps. I'm afraid that Siegburg's inhabitants were not curious enough to ask for details at that time...
Here: http://www.blattwelt.de/siegburger_blaetter/nr_8_bomben.pdf we have some information concerning the end of WWII in Siegburg, with an aerial photo of the city from march 45 (the hospital on the bottom right) - also only in german, sorry! Google can translate it via copy and paste, but the result is a bit odd.
If I find more, I'll write it here!
Christoph