SIRS (Systemic Inflammatory Response Syndrome): This is the most likely diagnosis given the elevated WBC count of 25,000 and a heart rate of 113 without an identified source of infection. SIRS can be caused by non-infectious conditions such as trauma, burns, or pancreatitis, and the absence of a clear infectious source aligns with this diagnosis.

Early Sepsis: Although there's no identified source of infection, it's possible that the patient is in the early stages of sepsis, where the infectious source has not yet been found. The body's response to infection (elevated WBC and heart rate) could precede the identification of the source.
Inflammatory Conditions: Conditions like acute appendicitis, cholecystitis, or diverticulitis could cause an elevated WBC count and tachycardia without an obvious source of infection at the initial presentation.

Occult Bacteremia: It's crucial not to miss bacteremia, especially in patients who might not show typical signs of infection due to immunosuppression or early stages of disease. Blood cultures are essential in these cases.
Endocarditis: Although less common, endocarditis can present with non-specific symptoms and signs of infection, including elevated WBC and heart rate, without an obvious source.
Neutropenic Sepsis: In patients with neutropenia, the typical signs of infection, including elevated WBC, might be masked. A high index of suspicion is required in these cases.

Fever of Unknown Origin (FUO): This is a condition characterized by a prolonged fever without an identifiable cause despite extensive diagnostic evaluation. While it's rare, it should be considered if other diagnoses are ruled out.
Autoimmune Disorders: Certain autoimmune disorders can mimic infection with elevated inflammatory markers and could be considered if other diagnoses are excluded.
Malignancy: Some malignancies can cause systemic inflammation and mimic infection, although this would be less common and typically associated with other symptoms and findings.