When the Doctor discovered that Rose was no longer in the booth he'd left her in, he'd spun about on his heels and craned his neck and looked under tables and behind the counter. And when she didn't appear to be at the cafe at all, he barreled out the front door and very nearly stumbled into Rose herself. He straightened and teetered a bit and he blinked at the squirmy little ugly thing wrenching in her hands.
"Whoa. Feisty little fella!" He laughed and patted the angry flailing plastic on the head, then leaned his hands on his knees to get a closer look. "Much better than a trash bin, I'll say. This is a full-blown auton: he's all plastic -- solid plastic, if ya feel the weight of 'im -- being controlled by a remote intelligence." He tapped his temple with a grin. "Fantastic stuff, just the link we need. Well done, Rose!"
But at the presentation of the little silver key, his expression fell. He smacked the lapels of his jacket and checked all his pockets before he submitted to take the key from Rose's hand. "This little bugger had this key?" The Nestene Consciousness must know what it's for -- which means he was being targeted. Which means the Consciousness knew where the TARDIS was. Which means --
The Doctor was pacing in a small circle, muttering to himself, so he barely heard her questions about Mickey. When he finally did come up for air, he took the little plastic man into one arm and grabbed Rose's hand with the other. "Mickey'll just have to find us on his own, we've got no time. We've got to go back to your apartment, now, and we've got to run. Come on!"
With that, they were sprinting along the roadside and down the quiet dusky streets, the Doctor dragging Rose along without waiting for an answer, back the way they had come.
"This key is the key to my spaceship," he explained as they ran, and he tightened his grip on the squirming little auton. "But why would the Nestene Consciousness need a key unless it had a lock?" This was bad. This was potentially very, very bad.
When they entered the courtyard of Rose's complex, the Doctor let go of her hand and barreled up the stairs, though it was Rose who had the means by which to get into the apartment at all. He was forced to wait for her, pacing, unable to stand still, holding the auton now by both arms with its feet dangling.
Inside the apartment, the TARDIS had gone.