I just hit 500 continuity checks on a single LRU without a single fault
It was a stubborn comms box on a Citation that had been giving the new guy fits, and finding zero issues after that many pins felt like winning the lottery, so what's your record for a clean sweep on a problematic unit?
That's a wild number of checks on a Citation box. Did you end up having to go deeper into the system wiring, or was it truly just one of those ghosts in the machine that cleared up after a reset? I've had a few where the fault just... vanished after a power cycle, leaving you feeling crazy.
Start with the wiring diagrams and check for any service bulletins, because sometimes a known bad splice or connector hiding up the line will mimic a box fault. Totally get what @cora_ramirez said about ghosts, but my record clean sweep was around 300 checks on an air data computer before we found a cracked pin in the backshell they only saw under a magnifier. Never assume the box is bad until you've ruled out every inch of wire and every connector pin.
300 checks though? sounds like you were just bored and didn't want to close the job out. most of the time it's just a loose connection or a bad ground that a reset would have fixed anyway.