Terezi is already out and in about, performing her investigative duties, as is required for her now that she's in a very unknown place obviously sent here by force with people containing questionable motives. So far, in person she's met individuals who seemed content with their lives here and it's made her very suspicious of actual person to person contact.

But, the Seer wants to experiment with some more interaction and so this is why when Heiji is charging out, Terezi deliberately makes it so he walks right into her. Especially with how quickly she seems to come out of nowhere.

"Rude! Don't you know to watch where you're going?"