Start with the basics. Most microwaves have 3 door switches. I've repaired a couple microwaves, and that was the cause. Then I'd suspect caps.
They can but they don't lol here's how to know for sure. Go look at a old Cband only mesh satellite dish , they receive at 4ghz a microwave signal. Not at the same power though.
However thier job is to not allow any microwave signal to penetrate through, rather to reflect microwave energy back into the feed horn.
We all know the higher the frequencies the smaller the wave, that's why your uhf and vhf tv antennas are huge or have huge dipole. Inside a C band feedhorn you'll find the dipole for reception of a 4ghz signal it's about 2 CM. (Sorry yanks , it's called metric, it's over 100 yrs old, time for you to catch up, stop using your barbaric biblical measurements .)
Taking into account that no hole in microwave oven is greater than that of mesh on a 3 metre satellite antenna
And that a 2.4 ghz signal like the one Panasonic magnetron makes . Also keep in mind that the USA allows 5 as a measure of leakage acceptable and Canada allows 1 where Australia allows 2. Not sure what the measure is it's in the Panasonic microwave oven repair guide though.
OR
Also you could look at a 2.4 ghz Wi-Fi mesh antenna and see that the gaps are huge yet it still is a barrier to that frequencie.