I had a very slow propane leak in my rig, and the gas system with the tank valve closed would not hold pressure in a homemade manometer like this one:
Manometer (https://www.komar.org/faq/manometer/)
I tried sniffing, and tightening things, and applying some of the blue leak detection bubbling liquid to possible leak points.
After a chasing a couple of hunches with no results, I took a chance on a cheap electronic propane leak detector for $26.
Gas Detector Alarm, Portable Natural Gas Tester Detector/Combustible Propane... (https://www.amazon.com/gp/product/B07H671NJ3/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1)
Obviously, I was skeptical that a $26 dollar product could be accurate when more reputable detectors cost many times that price. I watched a couple of youtube videos and decided to take a chance knowing that it might or might not help.
I was thrilled. It didn't take me long to pinpoint the leak a coming from the gas valve on my water heater. No amount of blue liquid or sniffing could have pinpointed that leak, and the replacement valve is expensive enough ($130) that I didn't want to just throw parts in to test a hunch.
If you have a hard to track leak, I think the inexpensive tester is worth a try. Be aware that you're not buying a high quality piece of gear. In my case, it did the trick. Having one from the start would have saved me a lot of time.
Rich
I agree that a "sniffer" is a good piece of equipment to have; I recently bought this one to confirm a propane leak at the tank service valve.
Y201 Propane and Natural Gas Leak Detector; Portable Gas Sniffer to Locate... (https://www.amazon.com/Natural-Detector-Portable-Combustible-Explosive/dp/B07FSVCSSL/ref=sr_1_2?dchild=1&keywords=lpg+leak+detector&qid=1607184090&s=hi&sr=1-2)
I smelled propane when I turned on the valve, and used Rector Seal "Better Bubble" daub on goop on the fittings to check for the leak location(s), but using a sniffer saved standing there, staring at the joint, often with a flashlight, to pick up tiny bubbles. The leak is now repaired (service valve replaced) and the sniffer is part of the essential equipment list!
Not a bad deal for about the cost of a pizza.... ;)
joel