very strange cooling system question

[Peter]

for the last 2 yrs.(30,000mi) on my 86 Jetta Turbo, every once in a while I get a little coolant leaking from under the cap (but not enough leakage to be continually adding coolant)...my mechanic who has many years of experience can only scratch his head...the system has been pressure tested to determine a head gasket leak, all hoses tightened up, new cap several times, I plugged the weep hole on the old water pump,...any suggestions or opinions would be appreciated