Didn't see your post until I had already posted. According to the Rules and Regulations, visitor parking is on the street only. It doesn't state how many parking spaces per unit, but that unit owners must park in their assigned spaces.
If another vehicle had been parked in one of Hicks' assigned spaces, I wonder if it belonged to a visitor other than Razan, but Hicks assumed the vehicle belonged to a visitor of Yusor and Deah. Maybe the vehicle parked there, if there was one, looked similar to Razan's vehicle or the vehicle of another frequent guest of Yusor and Deah. The "offending" vehicle could have been gone by the time LE arrived.
Granted I'm going off of the interview his sister did with CNN, she said there were a few visitors spots and the rest parked on the street. Adjacent to the alleged killer's spot was one of those visitor parking spots. She described Deah's space, next to his was the alleged killer' space, and next to that was one space for a visitor. He wanted it for his wife, she claimed. I will try to find it in wording. I'm not sure if she's unaware or misquoting someone else, but this is what she told AC.