No, you're certainly not the first biker that can't find their tank bag rain cover. What's not firmly attached is easily lost and nobody looks for their rain cover when the sun is shining.
Fortunately, rain covers can be bought separately.
A good rain cover will protect your tunnel bag and all of its contents against the wettest of weathers. The top is often made of a transparent material, so you can still see your map or smartphone.
After use, don't forget to store them well.