Remember during the time of the Joker's arrest there was still tons of stuff going on. There was probably no time for a complete procedural booking and lock up of Joker. I don't see why they'd wash his face, remember the guy with the cell phone? Granted that was underneath his clothing but surely you'd feel that lump while frisking him?
	
		
			
		
		
	
				
			 
				 
 
		 
 
		
 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		

 
 
		