Activated by notam means exactly that. If there is no NOTAM the you can legally plan through.
Mil airspace also has the disclaimer that it can be activated at short notice, and you can find yourself in the middle of their zone upon sudden activation.
Have had this happen at East Sale more than once.
The key is communication. The mil controllers will announce and then begin contacting 'targets' within the zone. They WILL work with you and get you clear of the zone with no fuss.
I've always enjoyed working with the mil controllers as they are very helpful.