On a shared pool and spa system, the spa sits higher, and a check valve keeps it from draining into the pool when the pump stops. If that valve sticks open, the spa empties into the pool overnight and the pool level rises. That is a valve problem, not a leak, although the two are easy to confuse.
If the spa loses water while the pool level holds steady, the spa has its own leak, in its returns, jets, light or the plumbing beneath its shell. Isolating the spa’s lines is how the two are told apart.
Still not sure it’s a leak?
Describe what your pool is doing, and we will tell you what it may be and what a visit would involve. If the water level is dropping fast, call us right away.