Thor's version of tentative is-- probably akin to a lion exploring an enclosure for the first time. There is hesitation without actual fear, indecision over true uncertainty, and so his attention is easy to snag, coming to a halt when he hears a voice direct enough to be for him.

An entrance? Thor looks up at the sky whence he came, then back at Ivan.

"I mean not to cause alarm -- this place appeared to beckon me as I flew by."