Clara decided that the Doctor was taking too long. She wanted to explore and find out why the TARDIS couldn't leave. She had to get back to Artie and Angie. Yes, she knew the ship was a time machine, but that didn't mean she was going to abuse the privilege. Something could go horribly wrong and she could end up stuck somewhere.
Well then. How ironic that just so happened to be the case.
With her arms crossed, she tilted her head at the ginger woman and smiled. Most people ignored the TARDIS - even here in Mandalus. So, imagine her surprise and curiosity when she saw Donna looking it over.