You are the X-Phile to see why Diana took off her shirt.

10.  She wanted to show off for the videotaped surveillance in Mulder's
apartment.
9.  She's tried and tried to make Mulder go off the deep end, but
nothing else worked before.
8.  She read in a book somewhere ("Whoring for Dummies", TM) that this
type of action may help rekindle a dead romance. Evidently it didn't work.
7.  CSM was also hiding out in the room but the camera didnt show that.
(He has the amazing ability to be in two places at once, hence the next
scene with the meeting).
6.  She was in Europe for so long that she thought it was customary, and
she was getting ready to go to the beach.
5.  She was hoping that Scully would come back and find her in such a
pose with "her man".
4.  She really has the hots for Mr. X. She doesn't believe he's really
dead, and thought the "X" in the window was a really tacky signal (she
learned about both Mr. X and the signal from Mulder). Her new "signal"
is more like a siren.
3.  She's trying to become like one of the featured "stars" in Mulder's
magazines, in order to win him back.
2.  Her shirt was too itchy. (LOL! I needed a nice one in for a change)
1.  She feels much more comfortable being loose. ;)