Unless you have a functional check-valve between your regulators and your kegs, there's a good chance the 16 psi you're seeing is the pressure inside the keg.
How it got that high is a whole 'nuther question. But to confirm that, make sure the regulator is dialed to zero (like just before the screw falls out
) then turn off your gas tank valve, remove the gas QD from the stout keg, pop the pressure relief valve on the regulator and note that the low pressure gauge falls to zero.
Then turn on the gas tank valve. The regulator low pressure gauge should still show zero. Assuming it does, now hook up the gas QD to your stout keg. I'm betting that low pressure gauge jumps up to the same 16 psi you saw before...