Lots of people newly discovering the entertaining games that raiders, like Carl Icahn, can play in hiding, ahem, their stock holdings in target companies until the last minute. In the case of Yahoo, for example, Icahn is sharing notes/stock/etc. with hedge fund manager and kindred spirit Jim Paulson.
Here is a nice chart on the subject showing how Icahn, Nelson Peltz, and others have used Morgan Stanley to help them quietly build some recent positions.
And why do they do this? It has a great deal to with antitrust rules and the FTC. As an activist investor — someone doing anything other than buying and holding a stock — you must disclose your position when you cross certain thresholds, and then ask permission from FTC to go further. That is slow, cumbersome, and noisy — you can’t exactly sneak up on those wascally wabbits — so fund managers are very clever at playing hide the stock, at least temporarily.