This should only happen on CyanogenMod-based ROMs.
In order to avoid having the Android Media Manager index all the files MyTrails uses for offline maps (thus wasting space and battery on every startup), MyTrails adds a .noscanandnomtp file to offline maps.
This in turn prevents MTP (which your desktop uses to connect to the phone) from seeing these files.
If you wish to transfer an offline map from your phone to your desktop (perhaps to share the offline map with a friend), you can:
- create a ZIP archive of the offline map (on your phone)
- or temporarily remove or rename the .noscanandnomtp file (using an Android file manager) which is inside the main directory for your offline map, then cause Android to re-index the directory, perhaps by rebooting the phone and waiting a few minutes
you will then be able to see the map (or its archive) through MTP.