A long time ago I heard about a heavy main who utilized a metronome to remind him to turn around for spies or other threats to him and his team/medic, and I never really got around to trying this out until recently. I haven't tried it in a competitive enviroment like a lobby or pug yet but I was in a koth_king valve server where I noticed there was a pretty good spies doing work on our team along with a scout that was constantly showing up behind us so I downloaded the bounce mentronome application, and went heavy to try it out.
It ended up working pretty well. I had it set to 8 beats per minute, which in theory would mean I would be turning around 8 times per minute but it usually ended up being a bit less than that due to specific circumstances like if I was in a demanding fight or I had just killed a spy or two. I found it quite satisfying to be catching spy or scout in the act almost every time I turned out. If I noticed that there were less threats showing up behind me or there were less spies on the enemy team I would turn the metronome down to around 5 beats per minute.
So try it out yourself and see how it works for you, especially if you're a heavy main. I think something like 2-5 beats per minute would be good for highlander where there's only one spy. Also, make sure you use bounce metronome or something similar where you can turn down the beats per minute really really low because most metronomes aren't going to go that low.
Also if you've heard any stories of heavies using this before let us know.