To our Russian colleagues:
Is it an accurate translation?

I'm not sure how well "mad" fits. I know it's the colloquial way, but seems rather strong to me, comparing. May be misassessing the expressive factor, of course.
I think the general definition of обидеться is something like 'feel wronged/hurt/offended (by smth), feel resentment (feel bitter)'; it doesn't necessarily imply anger though.

was mad=she took offense ?

So, maybe: She was offended due to something.
?
Or any other wording?

я "at" не увидел сначала.

тогда мне кажется, предложение правильное.

to be mad at smb - злиться/сердиться на кого-либо

Так злиться же, а не обижаться. Обида может быть сопряжена со злостью, но необязательно, может просто горчить.

или нет? "злиться/сердиться" и "обидеться" кажется не синонимы.

тогда можно исправить на "She was offended due to something"

Yes, this was my suggestion.
Ooney, at any rate, I hope you will also post the other (English) variants and their respective Russian translations.

Well, given that in this case the feeling manifested itself in a decisive action, that is leaving for an unknown period of time, I suppose "she took offence at something" would work; in actuality, since this is a very useful word which lacks a perfect English hit, I'd love to have several translations listed as helpful approximations, but it's difficult to be comprehensive...
"She was offended due to something" is a good descriptor, although seems like it may feel a bit technical to use in actual live conversation as much as we use обидеться.
I'll try to think up a few variations and post them, per your request.

OK, I'm changing my initial wording.
Thanks.

BTW, I usually use the American spelling. If you wish to add variants with British spelling (which is your usual orthography), feel free to do that any time.

Okay, thanks for that clarification tidbit.