Factors and Versus Tests

This is the situation:
Three bandits are escaping and hiding. The patrol would find the bandits.
I -the GM- declare a vs test: scout (bandits) vs scout (patrol).
At the beginning of the session I have set the weather on “Spring Rain” that says “spring rain is a factor in any test involving outdoor activitiy”.

So, the three Bandits roll the dice and get 3 success. Do I add the factor for the rain, as if they got 4 success?

Right or wrong, I’d have added the factor as bonus dice, not successes, in a versus.

Factors don’t generally apply to versus tests in Mouse Guard. And since the factor would apply equally to both sides, it can just as easily be omitted from the test and have the same effect.