Air carrier Captain reported the unintentional release of the aircraft's parking brake while waiting to be towed into parking. The Captain then realized the brake was not set and later realized there were special parking instructions that had not been reviewed prior to arrival.

Narrative

Arriving in ZZZ we were assigned parking spot 1 and another aircraft was parked at 2. I am and the crew were not aware if a plane is parked adjacent to your parking spot we are utilizing the tow in procedure. The Ground Crew stopped us and we set the brake and run the shutdown procedure and checklist. We all thought this is the final parking position and started securing the airplane. I must have touched the brakes and was not aware of the braking brake not being set anymore. The Ground crew came on the intercom and asked us if we released the brakes and I said no. Then I became aware of the parking brake not being set and set the brakes again. Apparently during connecting the towbar the plane moved and we were not aware of it. That's when the ground crew told us we will be towed into final parking position. After rereading the station guide; I realized that I had missed the part about the tow in procedure if a adjacent plane is parked to your parking spot. Not in contact with the ground crew and not aware of the tow in procedure if a aircraft is parked at the adjacent spot. Also I/We missed and skipped the tow in procedure as described in the Station guide. I am not completely sure if this is a new procedure or it been in the guide as I never been towed into these spots. Suggestions is always read completely the station guide even if you are familiar with the airport or your base.