As the Invisible Woman, Susan can render herself wholly or partially invisible at will by bending light around her.[50] She can also render other people or objects fully or partially invisible too, affecting up to 40,000 cubic feet (1,100 m³) of volume.
EDIT:
Invisible Girl
Initially, her powers are limited to making herself invisible. However, before long Sue discovers she can make other things invisible as well as create force fields of invisible energy.
https://en.wikipedia.org/wiki/Invisible_Woman
EDIT:
So I guess it takes a conscious effort for her to make something other than herself invisible, and in the case of the plug, she just forgot.
That’s what I think every time I see this comic, she’s forgotten to make it invisible
What a blunder I hope some one gets fired for this comic
Invisible by bending light.
You can’t bend light where thr sun don’t shine.
Got em
So this means she intentionally let it be seen and possibly wanted something even more rock hard? 😉