Air carrier Captain reported a temporary fume event during cruise. Flight crew completed the fumes QRH procedures and requested priority handling for an air return which culminated in a safe landing.

Narrative

I was towing an aircraft entering the hangar area and saw my wing walkers positioned at X specific spot to park up; we usually and preferably park the planes at [spots] Y; Z; A because is less difficult at those spots. I keep going with the operation; turned around and lined up the plane backwards; when ready I started moving forward and when getting into the taxi center line I had to stop; back up and call my left wing walker through the window because I couldn't see her in a certain moment; so I just wanted to make sure she was visible before continuing the operation; before getting there they made sure there was no equipment on the safety or diamond zone; they have everything clear when they started marshaling the plane in. I had the aircraft very well align on the line; following the marshal signals he had to bend over his body in order to see him as well; I was watching also my left wing walker. I keep going slowly and following the marshaller sign I stopped when he show me the X sign as a procedure when suddenly my left wing walker was running towards me giving me the X sign too saying that the wing touched one stairs positioned outside of the diamond zone which was supposed to be clear from any inconveniences. I asked why she didn't touched the alarm button incorporated on the safety wand and she said she wasn't trained to use those wands. Then I asked the mechanics why if the stairs are positioned in a safety area out of the diamond zone why did that happened? if there was any issue with specific aircraft that can be parked on that spot? Most of the time they want us to do some maneuvers such as bringing the plane in reverse to these spot that are more difficult to complete due to visibility but they still pushing us to do it due to their needs base on operation.