Schrader valves.
The service ports gauges connect to. Small, routinely overlooked, and a common slow leak.
A Schrader valve is the same spring-loaded core you have in a bicycle or car tyre, scaled for refrigerant. Air conditioners have them as service ports so a technician can connect gauges without opening the sealed system.
They sit under screw-on caps, usually at the outdoor unit near the service valves.
Why they leak
Because they are a moving part in a sealed system, and every service visit works them.
The core has a rubber seal that ages, hardens and takes a set. Connecting and disconnecting gauges depresses it repeatedly. Grit on the seat stops it closing fully. The result is a slow leak at a port rather than anywhere dramatic.
Schrader cores are a genuinely common source of the leak on a system that has been low more than once, and they are among the cheapest things on a system to replace.
The cap matters
That screw-on cap is not just a dust cover. On most fittings it carries a seal and is the primary seal; the valve core is the backup.
So a missing cap is worth noticing. It means the only thing holding your refrigerant in is a rubber seal in a core that somebody has been pressing on. If a technician leaves caps off, that is worth mentioning.
Why this appears on an honest site
Because it is one of the places a slow loss of refrigerant hides, and because “we topped it off” without a leak search is a repeat purchase.
A system that has needed charge twice has a leak. A core and a cap are trivially cheap to rule in or out, and checking them is faster than almost any other leak search. If that step was skipped before somebody quoted you a coil, the scam guide covers what to ask.