The Mac uses a system that reminds me of “Plug and play” to determine what display is connected, and what its capabilities are.
To get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display. "No signal detected" is generated by the DISPLAY, not by the Mac.
This query is only sent at certain times:
• at startup
• at wake from sleep — so momentarily sleeping and waking your Mac may work
• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go
• hold the Option key while you click on the (Detect Display) button that will appear in Displays preferences (from another display)
in addition, when a second display is no longer present, its opened windows are moved to the closest edge of the adjacent display, because otherwise they would become inaccessible.
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You did not issue user commands to tell your Mac the setup was changing. But adding/removing an external display is a Major change, as far as MacOS is concerned.