Motor problems...help...?

my daughter has a 1999 mercury villager.recently it has ran rough at times but not all the time.It was running rough ,her friends dad looked at it an it had a spark plug loose and it was low on antifreeze. he tightened the plug an filled the radiator an it ran fine the rest of the day.the next day it was running rough again so she checked the antifreeze an it was once again low so she filled it an it ran fine the rest of the day.i went across town an looked at it an found no leaks ,i replaced the once loose plug in case it went bad an told her maybe it was low on antifreeze again because her friends dad did not let the antifreeze cycle through. last night it was running rough again.i went this morning an looked at it to make sure the plugs were tight etc an i on top of the motor theres 2 plugs,i pulled the wire off an saw a very little bit on antifreeze around the spark plug on the block.so where would this antifreeze have come from? they didnt not take off the radiator cap when it was hot an the plug is nowhere near the radiator.so im figuring maybe a cracked block? no idea.no water in oil or oil in water thats visible to the naked eye anyway and as far as we have seen the motor doesnt overheat...thanks for any help.