As long as the frog is somewhere sheltered and damp then it should be ok. It must have been local enough to have got into the garage and so it should find its way back to where it came from, in due course, now that it is back outside.
When I was in London my cat 'found' our garden toad and decided to bring it in. Half way in she dropped the toad, which crawled back outside, and wondered about dragging her tongue on the carpet and foaming at the mouth looking very
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!
A quick call to the vets assured us that she would be fine, as would the toad, and to just give her access to plenty of water.