Not sure what the trick is, but I predict it's Haibara, who was disappointed Agasa couldn't make it. She also seemed a bit too happy to go get the stuff for the experiment before Conan told her to stay.
forgot to mention... yukiko could be vermouth in disguise since there isn't really any reason for yukiko to get the tablet. vermouth probably wants to get rid of her fingerprints from the yusaku deduction tv show case