Either you saved the guy's mobile number on your phone or he/they have saved your mobile number on their phone, maybe after you called someone who works there to ask something like whether they are accepting credit/debit cards or cash or what time they are open or whatever.
Even if you didn't share numbers either way, they might have it saved on their phone from when your number was owned by someone else before but I'm guessing it's not this case but the first one above - since you mentioned you visit them frequently.
Now the facebook/whatsapp apps (either yours or his) are reading your/their contacts and linking them on FB for friend suggestions and he has made a friend request.
It doesn't have to be mutual contact, even a one-way "link" can be done, especially if you have provided your phone number on the facebook website in your profile/security settings.[DOUBLEPOST=1487627724][/DOUBLEPOST]Oh, another avenue is that even though you claim "zero mutual friends" - how can you be sure? Maybe someone living nearby you, friend/neighbour etc who also visits the pump and who is also on your mobile/FB list has gotten linked the same way above and via that route you are seeing a friend request.