I'm leaning towards 10%, with the bit over that being explained by guaranteed every 10 adventures non-combats.
10%, with the tenth adventure being forced to be non combat if the previous 9 were not, is probably around 13% overall observed rate.
Edit - Simulating the numbers in Excel, unless I made mistakes, 5% with the 10th forced is actually around 12.5%, 10% with 10th forced is around 15.5%, and 15% with the 10th forced is around 18.5%.
So that'd suggest 5% based on our figures, and that you'd only see the non-combats every 10 turns with +5% combat, which shouldn't be hard to test.