Instead of having a ready-to-use car in their pockets, after enabling the checkbox Enable more realistic parking cims ordinarily try to park their car before reaching their target (up to 10 times, then they give up). This is the point where the Parking AI comes into play. But the base game does not come anywhere near real-life behavior when talking about parking and returning to/using a parked vehicle. If a free parking space lies in the close vicinity where cims want to get rid of their car everything is fine. The reason is that cims do not care much about parking spaces. This (mis)behavior is also known as 'using pocket cars'. You may have noticed that cims sometimes spawn a car at their current position or that cars suddenly disappear (especially noticeable at public transport stations).
