I think I may have found the source of my CO2 leaks. The tank pressure gauge on my primary regulator looks like it's malfunctioning - the pointer assembly appears to be sprung loose. I suspect that the leak is in there.
I went to Airgas to get my tank refilled (again) and took the gauge with me to see about a replacement. After some searching the guy offered me a welding gauge that has about the same pressure indication range as the original one, and off I went. There was one crucial factor I didn't consider, though: my gauge is left-hand threaded.
I got home after work and went to reassemble the regulator only to discover the thread issue. Obviously I'm going to return the new gauge, but that left my in a jam, since I still didn't have a fully functional regulator. I figured I would just plug the gauge port for the time being with a brass plug and worry about a tank pressure gauge later. (After all, the other one hasn't worked for quite some time, other than as a contributor to global warming.)
Oops. The plug would have to be left-hand threaded too. Guess what? You can't get a left hand threaded anything at Lowe's. Not only that, the gauges they have in the compressor and welder area are all right-hand threaded.
What I ended up doing is packing the gauge orifice with JB Weld to create my own plug. I put a little piece of plastic over the top to spread out the pressure from the tank (stuck in the excess JB Weld), taped it, and screwed it in. So far it appears to be OK. I'm taking the precaution of not leaving the tank valve open and cutting off the shutoff valves in the lines to minimize the leak possibility if I got it wrong. I'll return the gauge at some point.