MassDOT crews were expected to work through last night scouring hundreds of Big Dig manholes for potential problems after a 200-pound manhole cover went airborne inside the Thomas P. O’Neill Tunnel during yesterday’s morning commute, struck a car and killed an elementary school teacher.State transportation officials didn’t say last night if they found any other issues with the more than 500 other manholes, drain systems or electrical panels set into Boston-area highway roadways, citing the ongoing work.