A quick summary for posterity.
On locating a carrier:
"Few if any nations today possess an assured capacity to track carriers continuously. All of the relevant methods — radar,
electronic eavesdropping, electro-optical and acoustic sensors — have major drawbacks such as high cost, vulnerability to
preemption, and inability to precisely discriminate. While that may change over time, aggressors will still face a daunting
task in penetrating the layered defenses of a carrier battle group."
"The areas in which carriers typically operate
are so vast that adversaries would be hard-pressed to find them even in the absence of active countermeasures by the battle group...
The most practical way to conduct surveillance over such expanses is with satellites, which have a much wider field of vision than
sensors operating inside the atmosphere. However, China does not presently possess military reconnaissance satellites capable of
finding a carrier, and the commercial satellites to which it might turn take days or weeks to task and deliver imagery -- making them
useless for finding a continuously moving vessel."
On tracking it:
"In order to keep up with the carrier's movements, an attacker must establish a continuous track of the vessel using some combination
of land-based, sea-based, space-based and airborne sensors. Moreover, the track must be sufficiently precise so that it can provide
targeting coordinates to weapons when they arrive in the carrier's vicinity. As of today, even the United States has difficulty
accomplishing such a feat, and no other nation is close to having the requisite capabilities."
"Space-based or airborne radar with the capacity to track moving ground targets
(such as the Joint Surveillance and Target Attack Radar System, or JSTARS) has the greatest potential for providing reliable
targeting information, because it combines precision with a capacity to operate day or night in all kinds of weather. However, few
prospective adversaries are likely to have such systems before the end of the next decade."
Summary: Note that this article is circa 2001, so when they say "few prospective adversaries are likely to have such systems before the end of the next decade", I read that China and Russia could potentially have them now.
So I think there's a strong case for carriers being hard to track historically, but it sounds feasible that large nation-states have gained the ability to do so in the last 5-10 years, and it seems likely that that capability will be democratized further by low-cost commercial imaging (E.g. see SkyBox for the state-of-the-art in civilian high-resolution low-delay satellite imaging).