Losing clutch fluid

My 1988 B2200 is losing clutch fluid, was below the minimum level, and I cannot yet find the leak. I checked the slave cylinder, the two flexible lines, the steel lines, and the clutch master, even scrutinizing the firewall at both the engine and driver side to look for a leak, couldn't find leak. I don't know if the firewall is double panel and fluid could leak into a cavity (like brake fluid can on an old VW). I've had clutch masters leak before, so my firewall area paint is long gone.

I added fluid, cleaned up the engine firewall surface there, will keep an eye on level and check for leaks; pedal and clutch operation feels normal.

In the 28 years I've owned my 1988, I've swapped out the clutch master and slave cylinders about 6 times, though not since 2011. I've also replaced both clutch flexible lines exactly once each. I flush completely with every hydraulic repair. In January 1997 I bought lifetime warranty clutch master and slave cylinders from an independent shop which was bought out by CarQuest and later by Advance Auto; Carquest handled the warranties but the last time a woman manager said "no more", but then they were bought out by Advance, so haven't tried them yet for a warranty replacement.

So unless someone here has a theory where the leak could be, or I find something with a month or so, I'll likely just replace both cylinders again.