Although can not see who Darth Sidious is, he knows it’s Julian. What does he say he thinks the reason Julian changed his costume?

From the text:

The first costume I saw as I walked inside the door was Darth Sidious. It had one of the rubber masks that are so realistic, with a big black hood over the head and a long black robe. I knew right away it was Julian, of course. He must have changed his costume at the last minute because he thought I was coming as Jango Fett.