Curious how other companies handle mileage reimbursement.

Curious how other companies handle mileage reimbursement. I have worked for almost 15 years as a specifically remote employee - I don't even have a mail slot at the main office. I was hired as a remote full-time (and now exempt salaried) consultant with a company phone in my home office. On occasion, we have mandatory meetings at the main office about 30 miles away, and while I don't claim the travel time (because I am always over hours anyway--thank you, exempt employee status), I have several times submitted mileage so I can cover gas/wear-and-tear on my personal car. Recently, HR asked on what grounds I file mileage. It's not my usual place of business, so my understanding (for 14 years) has been that it's a reimbursable expense. How do other companies handle this?