Liron, plaster walls will not do much to microwave radiation but metal will. That's why microwave ovens have metal backs and a lot of internal metal baffles.
Actually, the early microwaves had some problems with doors sealing the microwaves in. By the time that the manufacturers had solved the problem, the safety agencies had written a whole bunch of laws about them that really did not apply much to the new models. But the laws and the warning signs remain.
As I understand your description, if the back or sides of the microwave is toward you, you are safe. If the front is toward you, then it would not be just on the other side of the wall.
The "power" of microwaves fall off at the square of the distance. If you are 2 feet away, there is one-fourth the danger as being 1 foot away. If you are 3 feet away, you are at one ninth, and one sixteenth at 4'.
If you are still concerned, Radio Shack has or had meters that you could get to check for leakage. They are pretty cheap but they may not still have them since there isn't much problem, however the ham radio guys might still use them. The CBer's did too.