Normally the backlight is on when the peripherals are connected to a
powered on computer and off when the peripherals are disconnected or
are connected to a powered off computer.
This is incorrect. Modern desktops (1) never turn certain power rails off (unless they are mechanically disconected from AC power outlet or by a back switch), and (2) bus power on USB peripherals is never turned off as well (laptops may have a slightly different power policy). So, USB ports are "always ON".
The "always-on" parts of PC mainboard run on low-frequency clock and consume relatively low power. This low-power "embedded controller" is used to sense PC buttons, and to wake up the PC when keyboard or mouse are moved or touched. Without power being supplied all time, this function would be impossible. Embedded WiFi also might cause periodic pings.
However, when going to "computer [soft] off", USB peripherals are placed into SUSPEND mode. It could be "selective SUSPEND", or "global SUSPEND". In SUSPEND, USB peripheral goes into low-power state, and only a "wake-up" function is available. Usually any connect of disconnect on USB ports also generates the "wake-up" event.
If your peripherals blink some lights periodically, it only means that the OS is doing some background bookkeeping, probably collecting some "telemetry", or checking for pripherals presence. Embedded WiFi also might cause periodic pings. If the USB wakes up and goes back to sleep, it is likely some OS service/task is causing this.