Somewhere I believe it was mentioned that apt #210 was the apartment that was used to show people that were interested in renting at Sawgrass.
If Casey had spent a great deal of time over at Sawgrass visiting Dante and Annie, maybe Casey was familiar with the office staff and hung out there as well.
The day that ZFG went to look at the apt, maybe Casey slipped into the office, saw the guest card on the desk, took note of the name and the name of her supposed daughters listed on the card and then waited a few minutes for her friend (the office buddy) to come back.
Casey gets bored or changes her mind and leaves the office undetected. She knows that the office staff showed ZFG the apt #210, noted the car that ZFG was driving and figured-hey! I have a scapegoat now.
Casey had been looking for excuses, names, an alibi, and scapegoats for at least a couple days at that point. For all we know, Caylee was in the trunk at the very minute that Casey first spotted ZFG's car pulling into Sawgrass that day.