It was in the map compare tool: http://project-gc.com/Tools/MapCompare
The cache in question was a traditional cache. It was a non-premium one, since neither of us are premium members at geocaching.com. My friend doesn't use Project-GC, I have premium membership. The cache was not disabled or archived. I had found it, he had not. The only box checked, was "One found", and I chose only country and region. No filters.
I thought it might be because of:
a) The person I compared maps with doesn't use PGC, and hence his statistics only updates once a week.
b) The cache was too young to show up. However I noticed that the cache in question was placed in September, and this was in November. Caches placed in October and November showed up.
c) I had chosen the wrong municipality, as the cache is placed close to the border between two. But when I chose to show caches from the whole region and left the municipality field blank, it didn't show up anyway.
My friend has found the cache now, so I can't see if my problem is solved or not.