Overflow pipes dont always exit the house at the nearest point... I got called to an overflow problem only a couple of weeks ago.
Very similar to what you describe .. "water dripping from pipe near ground floor window at rear of house" was the job description.
I checked all the usual places... the cisterns upstairs and downstairs. In the loft, both water tanks were fine.
Baffled I asked the owner if she had any other water storage tanks. ''Only the tank in the airing cupboard'' she said.''But that is in the front of house''...
When I checked... the storage tank was in a cupboard against the front wall of the house... (it was a combination tank,the top part of the tank is the storage tank used to feed the hot water tank)
Sure enough the ball valve was dripping.
Some bright spark had decided to route the overflow all the way through wardrobes and under floor boards and out to the back of the house instead of going through the wall beside the tank.
Maybe you have a similar problem..?