Lipoma or soft tissue tumor: Given the chronic nature of the swelling and the negative ultrasound for DVT and Baker's cyst, a benign soft tissue tumor such as a lipoma is a plausible explanation. These tumors can cause intermittent swelling due to their size and location.

Popliteal cyst (if the initial US was incomplete or if it's a smaller cyst): Although an ultrasound has ruled out a Baker's cyst, it's possible that a smaller cyst or one not easily visualized by ultrasound could still be present.
Lymphedema: This condition could cause intermittent swelling, especially if it's related to exercise or activity levels. However, lymphedema typically involves the entire limb rather than localized swelling.
Varicose veins or venous insufficiency: While DVT has been ruled out, chronic venous insufficiency could lead to swelling in the affected area, including behind the knee.

Soft tissue sarcoma: Although rare, sarcomas can present as a painless swelling and must be considered to avoid delayed diagnosis. Early detection is crucial for treatment outcomes.
Deep vein thrombosis (if the initial US was incomplete or if symptoms have changed): Despite an initial negative ultrasound, DVT should remain a consideration, especially if symptoms worsen or change, as it can be life-threatening if not treated promptly.
Infection or abscess: A localized infection could cause swelling behind the knee and would require prompt antibiotic treatment or drainage.

Cystic adventitial disease: A rare condition causing cystic degeneration of the adventitia of arteries, which could potentially cause intermittent swelling due to compression effects.
Neurogenic tumor: Tumors arising from nerve tissue, such as neurofibromas or schwannomas, could cause localized swelling and might be considered in the differential diagnosis, especially if there are neurological symptoms.
Ganglion cyst: Although more common on the wrist, ganglion cysts can occur in other locations, including near joints, and could potentially cause swelling behind the knee.