was sure vs. thought

Hi,
I am going make some sentences using the subject-noted words:
Scenario: Two people are talking about the missing pen. One person thought the other took it since the other seemed to have been the only one in the room when the supposed disappearance of the pen occurred (at least that is what he thought). Then, later found out that the pen was stuck in his briefcase, making it hard to see.

A: I think you thought I stole your pen, didn't you?
B: I was sure that is/was the case since you were only one in the room at the time of disapperance, but later found out, the pen was stuck on the bottom of my briefcase -- it was my mistake; I should have looked inside of the briefcase more carefully. Sorry.

As far as I can see, every verb is in the past tense except the underlined part. I feel the underlined part could either be in the present or in the past; I think the verb could be in the present since the subject matter is currenly related to the discussion and to people on hand -- unlike the cases involving the use of "thought," which will make almost mandatory for the verbs to be in the past.

Why is that if two tenses are OK? being sure of something and thinking about something seem to be a very similar mental activity.