I've heard it a few times, and would have assumed it was written ヘイ had I not seen it in writing. It seems to be an informal synonym of はい, used mostly in masculine speech. Also, it's generally pronounced as written, rather than as へえ like へい usually is. Is my assessment correct?
I only hope that I am thinking of the 「へい」 that you are talking about.
The 「へい」 I am thinking of is the 「へい」 that is probably more often heard in dramas and period dramas than in real life which is used by male speakers (at least in Kanto) who are being humble to their superiors.
It means the same as 「はい」.