Propane is heavier than air. You said the leak at the threads and pressure gauge is in a 2' diameter pipe at ground level, which suggests that leaking propane could settle into the pocket in that big pipe. The balloon usually goes up when there is a stack up of mistakes or conditions.
I'm curious. You said you did not think this was a dangerous situation. So why post the question asking for opinions?
I asked the question in my last sentence. This question came up as a result of my propane company notifying me that their insurance company was directing them to test all tanks for leaks. So I asked "Maybe I am I crazy(so tell me if I am) that I think this is not a dangerous situation. I gotta ask if any of you all have been faced with such a situation and is there a quick and fast drying sealant than might work?"
I think this is not dangerous because 1. The leaks are extremely small. 2. There is no source of ignition even if I had a heavy leak. Of course if I had a heavy leak I would be fixing it.
The large diameter/17" hard plastic tube sits sealed on top of the tank buried in the ground 30" deep. As the heavier than air propane fills that tube it then gets pushed out and above a 2" diameter hole in the hinged cover and it vents away into the air. In fact with the cover closed I can put my sensor at that hole and it will sense propane but a low rate. I open the tank lid and enough vapor flows away that I have to put the meter down into the tank below the area of the leak for the sensor to activate.
Finally where is the ignition source seeing that this sits in the ground in a flower garden 30' from the house?
And given the extremely slow leak just how much volatile gas is contained in that tube to cause a so called boom. Even above ground propane tanks with a direct fire rarely explode.
The vast majority of gas explosions occur inside structures that hold a large volume of mixed air and gas with an ignition source. None of these are occurring at my outdoor buried tank. I guess I could drop a match down there which would give the ignition source.
50 years ago I was a propane truck driver. Back then all furnaces had constant lit pilot lights with a thermocouple. I often was faced with the task of relighting a pilot light after someone had run out of gas after I filled the tank. I had the hair singed on my arm a few times as I held open the gas to the pilot light and enough gas filled the chamber to flash back towards my arms while I was trying to light the pilot light. I remember that very few pilot lights were easily accessible. They all seem back up inside the furnace making it difficult to light a match without having some small amount of flash fire happen.