If it's just a little metal in the box (probably just some foil for a side or something), then there shouldn't be any issue. Maybe a couple of brown spots from the sparks.
If it's from HokaHokaTei (or any bentou store) then it comes hot - so there could be metal. Ones from the convenience store are intended to be heated up in the microwave from the get-go.
Let's the two of us go halfsies!!Here you go - disposable chopsticks!!Okay, let's get right to it!!Okay then, I'll nuke it up in the microwave and make it piping hot!!NOOOOOO!!!BEEP!bzzbrkktbztk
BOM!bvwrrrrmmmmI-It's okay! I've got another one, see!It's okay!!I-It's all good, Onee-sama!!
There's not a problem whatsoever with the insides!!