No, microwaves cannot get hot when they're off and unplugged. The heating effect in a microwave oven is caused by electromagnetic waves, specifically microwaves, which are produced by the magnetron tube when the oven is turned on. When the oven is powered off and unplugged, there is no energy source to generate these microwaves, and therefore, there is no heating effect.
However, it's important to note that after using a microwave oven, the interior and exterior surfaces may retain some heat for a short period. This residual heat is from the heated food and the oven's components during the cooking process. As a safety precaution, you should avoid touching the interior or exterior surfaces of a microwave oven immediately after use to prevent burns.
But once the microwave oven is turned off and unplugged, it will cool down gradually, and there won't be any further heating or generation of microwaves until it's powered on again.