Shawn, I inspected a 25+- acre lake y'day. It was full of water. All around the back there was rusty water. I mean everywhere seeping through.
If I were you, I would have one worry, and one worry alone, and that would be the neighbor. If he's OK, you have no problem. It may be seeping through or into a spring, but you lake for all practical purposes, from your prospective is sealed. There is not real water loss. 1/4 inch a day is expected just about anywhere because of evaporation. It is nomimal.
