Which fans remain active continuously? Some fans will continue running until the printer returns to idle, but the auxiliary and part-cooling fans should stop once the print completes. The chamber fan can be configured to remain on after a job finishes; if this setting is enabled, it will not shut down automatically.
What material were you printing? If you’ve enabled enhanced fan settings, certain materials will keep the chamber fan running post-print to purge any harmful fumes from the build chamber.
You also mentioned hearing a fan noise just behind your X1 Carbon’s external spool holder—if I understand correctly, this is actually the small 30 × 30 mm MC-board (main controller) cooling fan, mounted on the controller heatsink just inside the left-rear panel (where the spool-holder bracket attaches when viewed from the front). It runs continuously whenever the printer is powered on (to prevent the electronics from overheating) and isn’t controlled by the Studio “Fan” menu, so it won’t shut off after a print completes.