Why does F-secure detect this application as containing a trojan, when I open the application - but if i run a scan on the .DMG file it does not detect anything..
This has happened on v2.92 / v2.93 and the latest nightly build...
Just about detection-name:https://en.wikipedia.org/wiki/KeRanger
About your experience with manual scan and launch/open application.
Maybe F-Secure solution on Mac with advanced technologies or detections (include reputation or rating). For example, like with Windows system - network-based HIPS like DeepGuard. Or its more fresh look with Ultralight design.
Potentially, launch or opening application will trigger more deeper analysis (also if manual scan was with suspicious state but with not enough points to trigger detection/notification). OR (based on wikipedia article) certain suspicious or malicious activities are detected during work. Even it may be only generic detection and, as result, false positive.
In addition, not a deeper analysis as always 'option' maybe based on performance meanings.