Pressure Used to Test Cooling System?

I own a '93 Chevy K1500 pickup with a 5.7L engine. Recently I had the intake manifold gasket replaced to stop an oil leak. 3 or 4 days after the work was done I found there was no coolant in the overflow reservoir but the radiator was still full, I guessed there may have been air trapped in the system. I called the shop and they wanted to do a pressure test to be sure there were no leaks. After the test was done no leaks were found and the mechanic told me that he had pressure tested the cooling system to 18 psi. This is 3 psi. higher than the radiator cap and now, 250 miles later, I can smell antifreeze outside the truck after the engine has warmed up. I can't see a source or hear a leak but from looking underneath the truck I can see a few drops of coolant that have collected at the back end of the block, between it and the torque converter cover. There is no sign of oil in the coolant or vice versa. Is 18 psi too high a pressure to use to test the cooling system and if so, could it have unseated a freeze plug on the back of the engine block? Does it seem more reasonable that the new intake manifold gasket is leaking?