Lymphedema: This condition is characterized by the accumulation of protein-rich fluid in the interstitial tissue, leading to swelling. The absence of redness, warmth, and pain, along with full range of motion, makes lymphedema a plausible diagnosis, especially if the patient has a history of lymph node removal or radiation therapy.

Cellulitis with atypical presentation: Although the classic signs of cellulitis (redness, warmth, pain) are absent, some cases can present atypically, especially in diabetic or immunocompromised patients.
Deep vein thrombosis (DVT) with atypical presentation: DVT can cause swelling without the typical signs of redness, warmth, or pain, particularly if the clot is not occluding the vein completely.
Venous insufficiency: Chronic venous insufficiency can lead to swelling due to the inability of veins to return blood to the heart properly.

Compartment syndrome: Although the patient has full range of motion, compartment syndrome can sometimes present with minimal pain or other symptoms, especially in the early stages. It is crucial to consider this diagnosis due to its potential for severe and irreversible damage if not promptly treated.
Neoplastic process: Certain cancers, like lymphoma or soft tissue sarcomas, can cause swelling without other symptoms. While less common, these conditions are critical to diagnose early.

Sarcoidosis: This autoimmune disease can cause swelling in various parts of the body, including the forearm, due to granulomatous inflammation.
Amyloidosis: The deposition of amyloid proteins in tissues can lead to swelling, among other symptoms, though it is a rare condition.
Filarial infection: In endemic areas, infections like filariasis can cause lymphedema and swelling, though this would be an unusual presentation in most clinical settings.