If you are referring to the two holes on the left, in shadow, just left of center in the picture, it appears they have bolts threaded from the transmission side.
Other than that, I do not see any openings for missing bolts.
It is highly unlikely that engine:bell housing bolts would cause oil leakage. They would have to be bolts that plugged a hole, or so loose the torque converter is able to move out of position if it was an automatic transmission. A manual has two sourced of oil, the rear main seal on the crankshaft and the input shaft seal on the transmission. There may be oil gallery plugs that could allow leakage if loose, but that is very unlikely.
Have you checked that your PCV valve is functional? If gooed up, it may cause more crankcase pressure than normal because of blocked flow, which in turn could tend to push oil out at the easiest place. Check that the PCV develops a slight suction on the inlet side(tube from the air cleaner housing to the engine) using a postcard held to the inlet. It should develop a slight suction at idle within a few minutes or less.
tom