Where does the XXX comment prefix in Eclipse come from?

XXX, along with FIXME and TODO, is known in Eclipse as a task tag, and is indexed by the IDE to let you find the spots marked with those tags easily. You can edit such tags in the Eclipse Preferences -> Java -> Compiler -> Task Tags.

As to where it comes from: it probably emerged form the "tags" that programmers spontaneously wrote in their code to quickly mark a given line. While FIXME and TODO are explicit enough, the reason XXX was used could be a combination of these reasons:

• The string "XXX" does not usually occur in regular source code and is easy to look for with tools such as grep or a simple text search in an editor;
• Traditionally, "X marks the spot" which needs attention; triple X even more so;
• The X key is very close to the Command/Alt/Windows keys and is easy to reach, being on the lower row of the keyboard.

I can't think of anything else...

From the Hacker's Dictionary entry for "XXX":

A marker that attention is needed. Commonly used in program comments to indicate areas that are kluged up or need to be. Some hackers liken `XXX' to the notional heavy-porn movie rating. Compare FIXME.

From Sun/Oracle's Java code conventions, section 10.5.4:

Use XXX in a comment to flag something that is bogus but works. Use FIXME to flag something that is bogus and broken.