In Terminator 2: Judgment Day, Inside Pescadero State Hospital, the T-1000 takes the "lucky day" security officer's shape just prior to slaying him. It then walks through the halls for a short time before reverting back to the default T-1000 shape. Why is this? Though the T-800 would have recognized the copy as the T-1000 in disguise, Sara and John would not have. And the T-1000, of course, had no way of knowing if John and Sara would be with the T-800 when it found them.
The official Frakes novelisation covers the T-1000's thought process. Officer Austin was chosen not only because of his status as a policeman but also because his physical profile was close to the T-1000's base form. Maintaining a different form requires additional energy that it doesn't want to waste:
The T-1000 walked forward. Because of the man’s obesity, it had been stressing its ability to expand molecularly by remaining in the Lewis form, so it had defaulted back to the more energy-efficient Officer Austin. It reached the bars. But it did not stop.
Its body began to divide like Jell-O around the metal bars. As it squeezed itself through like PlayDoh, its surface reformed perfectly.