Gas line pressure test
Even a small temperature change will cause a 5% change in pressure in your lines.
100psi is a unnecessary amount of pressure to subject a threaded system to, and I'm surprised it didn't actually cause a leak. Even welded lines only have to be tested at 60psi. Most jurisdictions require a 10psi test for 15 minutes, from my experience. Considering that the highest system pressure you can even have in a residence is 2psi, and most systems are 1/2 psi, your pressure test was overkill!
If you're concerned, test it again. But test it at 20 pounds or so, give it an hour, and if it hasn't moved you're in good shape.
Doesn't the leak detector stuff check for gas, not air? I thought it was for use on gas-charged systems that are thought to be leaking. I haven't used it myself. Soapy water does the trick for me.