'90 Accord post-repair coolant leak

In the past two and a half years I've replaced my '90 Accord's
radiator, water pump, thermostat twice, and just had the cooling fan
motor and temp sensor replaced after the car started getting hot even
while moving fast (without boil-over). Following that last repair, I
had to drive the car 150 miles home, following which I smelled coolant
and found a fair amount dripping from the radiator area while parked.

The temp gauge reads low-to-mid-level, as it should, and the receipt
says they did a pressure test (plus the mechanic said they'd idled it
for 45 minutes to be sure the fan was working). So why the coolant
spill? Any ideas? I don't know how pressure test and idle compare to
real highway travel. I plan to call the mechanic when they open
tomorrow, but any ideas from the crowd would be appreciated.