Even though they said they tested for microwave leakage,
and said there was none, I am not fully convinced.

So waste more money and buy your own microwave detector:

http://www.professionalequipment.com/xq/ASP/id.6/subid.189/qx/default.htm

Seems more likely this oven just vents itself differently than your old one.
Move the stuff on top to another location. Store stuff that won't get
'cooked' on there instead.