This would definitely get you into hot water at my field (Denver Centennial, KAPA), but the guy who picked you up was a bit of a DB, in my opinion.
Where? A large chunk of KAPA isn't a movement area. Runway 10/28 and anything west of and including Alpha is. You may freely wander damn near anywhere you like on foot, east of Alpha. And I have.
That said, you might get a visit from the Airport Ops truck.
The "taxiway" along 10/28 is not a movement area. Sight lines from the Tower can't see over the hangars along there, so it will never be designated as such, either. There's no way to positively control it.
It's kinda funny the tower even bothers with "taxi to parking" after a landing on 28. I'm usually halfway to the hangar when I hear that. Once you're past the line and off of 28, you don't need a clearance to go anywhere over there.
West of Alpha, grab a handheld and ask the ground controller for permission to walk out there if you need to. Have done that too. Not a big deal. Airport Ops truck might show up to give you a ride.
With appropriate insurance, and a legitimate reason to be out there in a vehicle, and a sticker on the vehicle, you can drive around the whole place, east of Alpha. Done that too. Never had any need to drive west of Alpha, yet.
The really sad thing at KAPA is their catering to the rich d-bags in the jets who think they're entitled to privacy at a public airport.
They've required *some*, but not all, of the regular photographers who like to stand just outside the airport fences in a few choice spots, to get both background checks and airport credentials.
It's better than banning them altogether, but it's a d-bag move, intended to stop photos with visible tail numbers of corporate aircraft from being posted to the Internet. They hide behind the "security" excuse and whisper "TSA" lovingly in the public ear to rationalize it.
The inconsistency is flat discrimination, and they'd lose if pushed on that one, but they'd just ban everyone if someone pushed that legitimate case, so the few folks who've been caught in that BS have simply played along.
At least they didn't charge the photogs for the background check.
I'd bet my left nut the two biggest whiners are the Denver Broncos and Liberty Media, although Liberty keeps their planes way down at the south ramp away from the riff raff.