I have a pool with attached spillover spa. I have noticed in the last couple of weeks that overnight the spa water level drops about a foot to a foot and a half from the spillover. Almost all the way down to the jets. The pool is six years old and until recently the spa had always held the water level right at the top when the spa and pool were off for the night. Obviously the water is leaking somewhere.
My question is would it be leaking back through the plumbing into the pool? I do have some hairline cracks in the gunite but those have been there for years.

Most often, this is a failing check valve at the equipment pad's return plumbing to the spa. Next is the valve that controls whether the spa suction is on or not. Last, but not least, an air leak somewhere.

Scott

I knew there had to be a check somewhere and was looking for the type check I am used to. I found it I think. It's black and round with a clear removeable plasptic face.
As Buffett says it was so obvious it plum evaded me.
Hopefully there is a rebuild kit so I don't have to replumb.

Your description sounds like it is a Jandy swing check valve. 8 screws to remove the clear top and you can pull the flapper out (watch for water pressure when you remove the top!). I take the spring off of these and let the water pressure itself close the valve.

They are great valves, and you can try and clean it (or maybe there is just something stuck in there and holding it open) or replace just the flapper. The housing should be fine.

I took the cover off and the gasket on the check and everything looks ok. Doesn't mean it is.
I am confused though by the plumbing and have attached a couple of photos of it. I have two supply lines to the spa. One of them has the check on it and the check closes in the direction the flow would be going to supply the spa. In the photo of the check the arrow is on the bottom pointing down. The flapper closes in the direction of the ground.
When spa mode is on pressure would be on both of these lines with the line on the right with the check being closed.
All the diagrams I have pulled up show only one supply line to the spa with the check inline on that. Why two lines? Shouldn't the check be behind both of these lines between them and the valve? I am guessing both of these lines are supposed to supply the jets in the spa? Why would only one have a check?
When the spa drains overnight the valves are all in pool mode. Does that suggest one of the jandy valves is leaking and allowing the spa to drain back to the pool through the drains in the spa?