Holley issue

If the pump has a spring that creates too much pressure, you'ld see it as overfilling bowls. That as you know would in turn result in in high level when you opened the sight plug and dripping from the boosters (or worse).

The throttles are below the seam with the main body. Off the top of my head I can't think of any passage on the secondary side that would let fuel come through there even with a bad or incorrect gasket. If I think of one I'll post. Its possible there's casting porosity.

The most likely explanation I can think of right now is fuel condensation. If you don't see it while running, but only when shutting down, give that some thought. The plate is cool, air is no longer flowing. Vapors condense out onto the surface.
Some posts about that at RFS. Here's one.
Capillary Action?

Here's another, in this case when engines are running.
Wet Palm