Sudden Frogs

When I opened the sliding doors to the wall garden this morning a little frog instantly hopped into the house and, being a rather quick-footed little bugger, whilst I am a slow old bugger, proceeded to hop across the floor and under the bed. I don't mind that part, as the doors are generally open the day long and he will pretty surely find his way back outside again. What bothers me is how he got there in the first place. I know I've mentioned this frog issue before, but it continues to perplex me. The wall garden is a narrow rectangular space just at the back of the house. It is bordered by walls that are about ten feet high. So how did the frog get there to begin with? I have theorized in the past that perhaps they come down with the rain, being delivered in such manner somewhat the way babies are delivered by storks. Another theory may be that they simply grow out of the ground, like weeds or vegetables (although, to be clear, I've not seen a vegetable spring up of its own accord as yet). As far as I know, frogs need a pond to hatch in, right? Then again, maybe this is not a pond sort of frog but more of a tree frog sort of frog. For that matter, do tree frogs hatch from ponds or from trees? As I've said, it's perplexing. I guess I'll just enjoy my coffee and forget about it for the time being.