Kegging Newb, must have a leak

I am losing CO2 at a rapid rate and not knowing where or why. I kegged for my first time a couple weeks ago and carb went great, kept it on the gas at 12 PSI with no issues, no loss, nothing.

I kegged a second beer a couple weeks later and force carbed it at 30 PSI for the day. (seperate from the 1st keg) I was going to just charge it a few times that day but left for work with it on gas at 30 all day. That drained the CO2 tank. I had initially checked for leaks (star san sprayed everything) and saw nothing.

I re-filled the CO2, brought both kegs out of the fridge and hooked up a T bar to add both kegs to the gas. I checked the T bar under water, no bubbles... star san sprayed everything.. no bubbles. Tank / Regulator... no bubbles. I hooked up both kegs at 12 PSI and for 3 days I saw no movement in the regulator and thought whatever leakage I had I must have tightened up. Skip to tonight, the tank has about 1 pound left and I noticed the low pressue gauge was tipped to 20 PSI (SWMBO says she was not in that fridge all day).

WTF can I do to check this out? Ideas, suggestions, comments, insults.... all welcome. Thanks...

It sounds like you've checked all your fittings. 90% of the time I get a leak, it's in the hose connections or the threads on my regulator. If you're certain you have no leaks there, then I'd fill your kegs (while empty) with co2 and submerge them in a bathtub full of water. This will show you where your leak is.

After further review; the picnic tap (a suspect one to begin with as it hosed me down the first time I tapped it) was slowly leaking and left me a little (thank god a little) puddle of beer but was bubbling for sure. Unhooked it, throwing it in the trash. To the LHBS I go.