Friday i added 3 POI on mobile with level -1 as indicated on this elevator i have taken : https://www.openstreetmap.org/node/11124711253/history

when checking now with Josm i see that the indoor footway i have take is indicated to be -2 in osm  : https://www.openstreetmap.org/way/513669186/history  (V4 seems more correct) idem for the escalator going to the metro. It's clearly indicated on the elevator that metro level is -2. 

Maybe SNCB use another level tagging but in that case where is the reference ? I just taken this entrance and not checked another one. 

Less problematic ; i have the impression that https://www.openstreetmap.org/way/1199403256/history is a little bit more to the north