Hitachi HIT BG17SS Instruction Booklet page 9

Steam
When you microwave food, steam is created as water in the
food heats up and evaporates.
The steam formed during microwave cooking is entirely safe. It has
to escape from the microwave to prevent pressure from building up
inside the oven. It does this through the vents on the back of the oven
and from around the door.
Condensation
As the steam cools, you may find the condensation gathers in small
pools inside your microwave and on the work surface beneath the
door and vents.
Always dry the oven after cooking foods with a high moisture
content.
Your microwave is designed to let steam escape from the vents
and around the door without microwave energy escaping.
There is no danger of microwave energy escaping with the steam.
Introducing microwave
cooking
GB-9