It is generally safe to leave covers or lids in an "off" microwave. Microwave-safe covers or lids are designed to withstand the heat and microwave radiation during the cooking or reheating process. Once the microwave is turned off, there should be no further radiation or heat emitted.
However, it's always a good idea to refer to the specific instructions provided by the manufacturer of your microwave and the covers you are using. Some microwave-safe covers may have specific usage guidelines or recommendations.
If you're uncertain about the safety of leaving a particular type of cover in your microwave, you can check the packaging or look for labels indicating that it is microwave-safe. If you can't find any such information, it is best to avoid leaving it in the microwave when it's not in use.
Additionally, it's worth noting that certain materials, such as metal or aluminum foil, should never be left in a microwave, even when it's turned off. These materials can cause sparks and potentially damage the microwave or pose a fire hazard if exposed to microwaves during operation.