I know this may seem a silly question, but here goes. I have a Fabia Mk4 Colour edition. It has 2 USB ports just in front of the gear lever which I use, (occasionally), to connect my phone for sat nav and music via a USB cable - (Android Auto).

If I were to leave the cable inserted in the USB port, but just unplug the phone from the other end of the cable would there be any danger of battery drainage or other adverse issues ??

No. There is nothing in that cable except wires.

....Unless it is a USB hub connector - which connects several USBs, which might then have a chip in it with a miniscule power drain -- but it doesn't seem you are using that.

Just did a test - (Should have done this before I posted query!)

Plugged my phone into the USB port using cable with the ignition switched off. Phone didn't start charging. Switched on ignition, phone started charging. Think this proves that power only available through the port and cable with the ignition on.

Interestingly though, if I plug my phone back into the USB port after I've switched the ignition off, then it does start charging again, but only for a short time - about 15 secs. Looks like power is available for a short period after ignition is switched off.

I found early on when trying to find a workable arrangement for my sat nav that the power output from those usb ports is low. When using my large screen Garmin sat nav power has to be supplied from the cigarette lighter port hidden in the front arm rest or the brightness of the sat nav screen is limited. That socket is permanently live and provides more power than the usb c ports being referred to. I don't have the sat nav lead plugged in unless I'm going to be using it but it is stored inside the arm rest.