This article is from the X-Men Comic Books FAQ, by Kate the Short (email@example.com) with numerous contributions by others.
After the X-Men died in Dallas during Fall of the Mutants (UXM #227),
they were resurrected by Roma via the Siege Perilous. One of the gifts
Roma gave the X-Men was that they were invisible to electronic scanners,
television cameras, and so forth, to better help cement their reputation
This power served the X-Men well enough during their Australian days
(although, for no explained reason, the computers at the Reavers base
were capable of detecting them, and the X-Men themselves wondered why),
but soon after they went tumbling through the Siege Perilous for a
second time it became apparent that the X-Men had lost their
"invisibility." There being no other explanation for this power loss in
the first place, it's generally assumed by the racmx crowd that Roma's
spell wore off.
Of course, there's a more logical explanation of why and when the X-Men
lost their invisibility to scanners, as explained by Chris Claremont at
the 2000 Wizard World Chicago convention: They lost the power somewhere
between pages 10 and 11 of UXM #279, or around page 1 of X-Men #4. This
was a reference to Claremont's last work on the titles, so apparently
*he* had planned to still have it going when he left the books.