As it happens, I just fixed two of these - same problem on both. The problem is a poorly built (i.e. very cheap) keypad switch assembly. What you are looking at from the front is just a thin plastic cover (with the pads embossed on it). Behind that is the actual keypad, which is an array of small metal domes. They take very little force to depress, frequently deform, and eventually one "sticks." That is, it stays depressed, even when not touching the microwave front panel.
[You ''might'' could temporarily fix the problem with a hair dryer on low heat. Warm it up (without melting it) and it might pop back - Temporarily!]
The maintenance manual is available online from Panasonic. It's free, and almost worth the cost. You can buy replacement keypads from Panasonic and/or other dealers - about $40. Replacement requires no special skills, but a fair amount of patience. I am considering at least a quickie writeup on how to do it, given that the need is so common. Meanwhile, if you want to try it, please feel free to drop me a note with any questions.
Note: The "taping off" of one or more switch PCB connection may or may not work (depending on which switch is stuck). Worst case, wind up needing to replace the keypad, which you do anyway.