(1) Some books give this example to study:
I saw him cross the street. = I saw him start and finish that action.
I saw him crossing the street. = I saw him in the middle of the street, but I
do not know if he finished that action. (My telephone rang, so I turned
my head away and did not see what happened.)
(2) Therefore, I think that:
He was seen to enter the building. = He definitely went into the
He was seen entering the building. = He started to enter, but it is not
known if he actually went inside.