Over 40 years finding leaks in vinyl, concrete & fibreglass pools & can't find a leak in my own spa pipes.Over summer stored tub on a wooden deck while getting a cement pad & gazebo built.When I moved it onto cement pad I noticed a pile of small chunks of foam under wood deck. Filled tub & it sat for 3weeks waiting for electrician. no leaks. Once running & jets on water started pouring out from under tub.A steady stream only when running thru jet pipes. Drained & raised side. Leak coming from pipes along series of MICE tunnels. Have dug out over 2 feet of foam cannot locate a leak . How can I test this maze of pipes to find leak?

Logic suggests that were leak a result of mice chewing pvc flex pipe it would also show as a static leak during those 3 weeks of filled and not running.
Some above ground spas use clear tubing for air injection and venturi. I've also seen them leak when a jet was plugged and water backed up airways.
I've done my share of hacking away foam to find a leak in a spa.
You certainly sound qualified to find this after 40 years in the business.

this is the part in all the forums where some fool says "hire a pro" good luck in your search, it doesn't sound fun

We had similar challenge, except it wasn't nice cuddly, friendly mice, it was smelly pest carrying rats. All the foam had to be stripped, after the pest man had been, but it still left us with dead, decaying rats!
They had chewed there way into lots of pipe work that didn't become evident until we ran the system.
You need no sense of smell and plenty of disinfectant for this type of job, and a customer who doesn't mind paying!

Well this is interesting. I'm on here trying to absorb as much as I can about the pool leak repair business as its a service I would love to provide and know little about and here you are with a leak in your personal tub. Here is a rare opportunity for me to actually post something relevant and I hope helpful advice.

Fully Foamed hot tubs = arduous, arm and finger muscle building opprotunity! My experience has been this. I dump red food coloring in the already heated water, run pumps for at least 30 mins. Now go digging where you suspect leaks to be. The dye will stain the foam around the leaking fitting(s). Very few tools work well to remove the foam expeditiously. I like my large 5-in-1 painters tool, short bladed small pvc saw, rust remover tool (w/ sand paper flaps on it) chucked into extended shaft in my cordless screwgun and the ultimate tool for which I have found no real substitue...[my barehands]. In some instances I've had to quickly pump out the water, stand the tub up on its side, peel off the bottom of the cabinet and start digging in the foam to find the red/pink stains, set the tub back down, fill it, observe the suspect area, test, repair, repeat as necessary. I only do that to tubs where i have the room to do this type of exercise. Its an all day affair for two guys, and its almost 1/2 the cost of a brand new tub, when we're all done w/ the invoice. You are in for a fun-filled weekend project.

As far as the mice/rats go. When you're all done, throw half a dozen mothballs throughout the cabinet and button-up/sealup any opportunity to prevent the rodents from moving back in.

-Jeremy
I fix broken pools, spas, and hot tubs
Florida Leisure Pool & Spa
www.FLLeisure.com

Rodents don't like mothballs then?

I find using a high pressure washer works best, when the pressure is set right it will cut through the foam leaving the delicate pipe work intact.

oh man, what a fantastic idea! I've got to try that. I've also got to get my hands on a pressure washer.

raise spa and put on blocks or timber . make sure you have under center of spa to help support . refill spa ( you have to have room under to explore) and look where water is dripping out run spa if need be.