What explains the remaining $3.28 move? Yahoo also owns a 36 percent stake in Yahoo Japan, but that company's stock has traded flat in the last 10 days.
Read MoreStarboard's real plan: A Yahoo Inc. without Yahoo
That leaves two likely explanations. One is that investors have assigned a higher valuation to the core Yahoo business, which is mainly search and display advertising. Yahoo recently came under fire from activist investor Starboard Value, which criticized the company for spending too much on fruitless acquisitions and a bloated cost structure.
Yahoo CEO Marissa Mayer last week defended the company's strategy during a quarterly earnings call. In her presentation, she pointed out the value of many small acquisitions Yahoo has made and highlighted office closures and the elimination of contract workers. Some investors may have begun to think the company will embark on a shareholder-friendly strategy around the core business.
Another possibility: Investors have begun to give Yahoo more credit for the potential to divest its Alibaba and Yahoo Japan stakes without paying taxes. On Yahoo's conference call last week, CFO Kenneth Goldman sounded encouraged about selling Alibaba without incurring a tax bill. "I have used the word 'optimistic' and I'll stick with the word 'optimistic' that we can effect a structure that makes good economic and tax sense," he said.
The tax bill could make a huge difference in how investors value Yahoo. Starboard estimated in its letter to Yahoo that a tax-free divestment of the Alibaba and Yahoo Japan stakes could save the company $16 billion. Similarly, Piper Jaffray estimated in a note last week that the company's sum-of-the-parts valuation is $56 per share assuming no taxes versus $41 per share with a 38 percent tax rate.
Read MoreActivist's plan aims to save Yahoo $16B in taxes
There remains a question over when Alibaba could be divested. Yahoo has agreed to a one-year lockup preventing it from selling its Alibaba stake through next year, though Starboard's plan may be able to avoid such a restriction, according to a person familiar with the matter. Yahoo and Starboard declined to comment.
In any event, Yahoo's recent price action suggests that there's more than Alibaba's stock to watch closely. The company's tax bill has also become a moving target.