X-Files question: What are the taped "x"-es about on the windows in several episodes?

Question

I'm sorry I can't be more specific but in several episodes either Scully or Mulder will go into some darkened apartment somewhere and immediately go to a window and stick a masking tape "x" on it. I think these episodes had something to do with the long-term alien/e.t. plotline somehow. Does anyone remember these little "plot" twists and what they were about?

Answer 1

It's when he wanted to meet with that old guy that was giving him information. That was the sign that they used to call a meeting.

Answer 2

That was the way Mulder and Scully used to contact their sources inside the conspiracy, or those high up enough to know (Deep Throat, X, etc). If Fox needed to get an answer or meet with someone, he puts up the "X". The contact would always be on the lookout, so if the X was seen, they would meet somewhere.

Answer 3

Mulder put the x on his window when he wanted to make contact with his sources. He didn't have a phone number or anything and that was the only way to make contact. He'd tape the window and wait to be contacted. Scully only did it because Mulder was missing and she taped Mulder's window.