I'll agree with everyone here: most likely Kid candidate falls under Kogoro, Sonoko, and Sera.
Sonoko is especially fishy given her restroom comment. Plus, like some have mentioned, I have yet to see Kid dressed as Kid out of usual Kid case main cast.
Though, that doesn't mean we can completely rule out Kogoro and Sera.
Kid isn't familiarized enough with her
The argument that Sera is too new is kind of weak. He has already seen her on the train. Plus, when Kid first appeared in DC, he disguise as Ran. He didn't know Ran well enough then, but still chose to disguise as Ran.
Hiding the turtle either behind the fake aquarium wall or rock is quite plausible. Kid has done similar trick in the clock tower story in MK when Kid hid the arms of clock behind a fake clock face.
Can't wait for the next chapter T_T
I'm anxious to know what message Ran received and from whom she received it. (my gut's guess is Ran received it from Kid)