"Approach to asking the reasons of their visits"

I was heading to Edinburgh Royal Infirmary for an appointment. I was stopped by the parking attendants, I rolled down my window expecting to have a pleasant conversation to why I was there.

Their first comment was that I wasn't able to park here, unless here for an appointment. I responded with saying I do have an appointment with such and such at this time. They then proceeded to ask if I had my appointment letter on me. I didn’t have my appointment letter on me and I know I should have, but even if I did I didn’t want this random person to know my name, address, date of birth and the reason of my appointment due to personal reasons. 

They responded saying I wasn’t allowed to park at the hospital without proof of my appointment and had to then park in another car park as I wasn’t allowed in the patient one.

I understand these people have a job to do, however asking to see proof of appointment with all the personal information on the letter is a bit rude and then saying I had to park somewhere else.

Definitely need to find a softer approach to asking the reasons of their visits without forcing people to show their letters which can be embarrassing to the person.