Shares of Avis Budget Group (NASDAQ: CAR) are sharply higher Monday and significantly outperforming rival Hertz Global (NYSE: HTZ), which saw heavy insider buying today. CAR last traded up 6.7%, while HTZ is up 3.1%.

One reason traders are pointing to is the recently raised stake by activist hedge fund Marcato Capital, brought to attention by Friday's 13F. Marcato disclosed that it raised its stake to 3,963,000 shares from 3,343,560 shares in the quarter. The firm has since raised its stake to 5,717,175 share, or 5.4%, as indicated by a July 13G filing.

Marcato Capital has been involved with the stock since the second quarter of 2014, when the firm initially showed a 2,050,201 share stake. It has been methodically building the stake since then.

The firm's investment in the stock is currently classified as "passive," as indicated by Marcato's July 13G filing. The filing indicated the company has no current plans to influence management. However, as the stake grows and the stock languishes, it appears it's a matter of "when", not "if" the fund will go "activist."

Marcato is no stranger to activist campaigns and has run them in Lear (NYSE: LEA), SOTHEBY'S (NYSE: BID) and Bank of New York (NYSE: BK), among others.

While it is unclear the firm's cost basis, the stock is down 21% since June 2014. So, the firm cannot be happy.

Marcato is not the only one that sees value in CAR. In July, Deutsche Bank upgraded the stock to Buy, saying sentiment is too negative and, despite concerns, alternative rental and car sharing are unlikely to have a material impact on Avis's business model longer term.

"We believe negative sentiment on CAR has reached extreme levels and the stock now reflects incrementally bearish expectations for pricing growth, despite easier comps in 2H and potentially more rational industry-wide fleeting," analyst Chris Woronk said in the July upgrade. "We like the risk-reward setup and view CAR's free cash flow yield as too compelling to ignore. We also see an underappreciated upside case in Europe, part of which is a CAR-specific self-help story. With these catalysts in mind, in the face of increasingly elevated short interest levels, we are moving off the sidelines. Our $60 PT is based on undemanding valuation and implies 41% upside potential,"

In addition to Marcato, Viking Global and Glenview Capital also raised their stakes in CAR during this past quarter.