"The Hidden Machine Age" isn't a widely recognized standard term in history or technology, so it is possible you might be thinking of one of a few different concepts or a specific (perhaps slightly misremembered) title.

The Ghost in the Machine (Arthur Koestler, 1967) This is the most famous title that sounds similar. It is a book where Koestler argues that the human brain has built older, more primitive structures (the "ghost") into it that conflict with the logical, rational mind (the "machine"). The phrase has become a common idiom for the unseen intelligence or soul inside a physical system.

The Hidden Machine (John E. Pfeiffer, 1952) This was a popular science book from the 1950s. It dealt with cybernetics and communication theory (the work of Norbert Wiener and Claude Shannon). It explored the idea that communication and information control were the "hidden machines" driving biology and society.

The "Invisible" Machine Age (Metaphorical) You might be using the phrase to describe the current era of technology. In the Industrial Age, machines were big, loud, and visible (steam engines, factories). Today, we live in a Hidden Machine Age where the machinery is invisible:

• Microscopic: Microchips and nanotechnology.
• Algorithmic: AI and code running silently in the background of our lives.
• Remote: The "Cloud" (massive server farms) that powers our devices but is hidden from view.

The Machine Age (Historical Era) If you are looking for the historical period, it is usually just called "The Machine Age" (roughly 1880–1945), characterized by mass production, the assembly line, and the rise of mechanized warfare.

Which of these sounds like what you were looking for? If you can remember a detail about a specific author, a plot, or a context (like AI or history), I can give you a much more specific answer