While speaking to the receptionist at Pescadero State Hospital, the T-1000 glances into a mirror and sees police officers and Doctor Silberman approaching. It then morphs into the floor, leaving the receptionist befuddled as to where it went so quickly.
Why did the T-1000 vanish? I originally thought (and the movie seems to suggest) that the T-1000 felt that the police officers could identify it as an imposter, and it didn't need such a hassle. However, after I made such a claim on a different website, I was told that, given the huge amount of police officers in Los Angeles, it would be utterly impossible for these officers to know, just by looking at the T-1000, that it was an imposter. Nor would they randomly "interrogate" the terminator for no apparent reason (perhaps in Hollywood, but not in real life).
I guess you could say that the T-1000 was hoping to "luck out" with one of the officers walking on the floor where it hid, as it happened in the movie, of course. But acting in such a manner seems unnecessary for the terminator; if the officers gave it flak for whatever reason, the T-1000 could have slain them all in a few seconds.