Dear Laura Lopez Van Dam,
I don't quite agree with Jan, but i'll look at it in another way
2 major situations occur
a- your ancestors lived in Belgium before they emigrated : they will be in the normal BMD records of the city/municipality they lived in before 1870, their emigration records can be found in ships manifests and immigration records at their port of arrival and in the records where they lived.
the only exeption is that if your ancestor was belgian woman and she married a foreign national before emigrating, then she (and her husband) will have immigration police records
b- you ancestors lived in neighbouring countries and emigrated through antwerp : there will be 2 cases
b1- they arrived a few days before their departure in antwerp, and stayed in a hotel for a few days, then you will not find an immigration police record unless someone got involved with the police (drunkenness, fight, theft or other petty crimes) then you might find a record in the immigration police records
b2- they arrived a few weeks/moths before their departure, then an immigration police record will be found, especially if they did try to work to cover their living expences or cost their fare.
That is my experience from browing an odd 50000 of those records
