Is illegal work while on sick leave or retirement in Western countries considered lawful (halal) or unlawful (haram)?

Whoever enters the land of disbelievers with a guarantee of safety must respect their laws, as long as they do not contradict the Sharia (definition), based on the Prophet's (peace be upon him) saying: "Muslims are bound by their conditions regarding what is permissible." Therefore, illegal acts are not permissible.