No, they aren't thinking of another girl, they're thinking of Madeline. I know because I dug into it and I think the same. All the evidence shows she died in the apartment, likely falling head-first down the back of a sofa trying to reach for a window while she was dosed-up on sedatives by negligent doctor-parents (doctors? negligent? noooo).

The evidence (namely sniffer dog) points to her having died in the apartment, her body stashed (theory is fridge/freezer) and moved later. The apartment and the hire car were the only two places specialised sniffer-dogs found her scent. The problem is though, the dogs picked up the scent in the car which was hired AFTER she'd gone missing. There is absolutely nil evidence of abduction, no forced entry, no contemporaneous witnesses, nada.
Also note the parents litigate anyone who publishes a narrative where blame is placed on the parents. The fact they haven't been arrested (yet) is astounding to me.

https://www.youtube.com/watch?v=diz3V_LUwBo
https://www.youtube.com/watch?v=TB-GrSPbQQY