You nearly cost me Vienna, damn you!
When a battle is about to start and the "attack/autoresolve/withdraw" options are shown, there is an additional button "find drop-in for this battle".
This is only available if the battle is nearly balanced though (can't remember whether it's not present at all or greyed out if it's not an option).
I tend to have drop-ins for mid-sized battles mostly.
So, even if you have activated drop-in for your campaign, you have to actively select it per battle; don't worry, I don't think you'll miss it if it's there.