Can I leave realistically leave Belgium on a residency permit?

Hello,
I am an American citizen living in Belgium.  I just successfully finished the process of attaining a Belgian residency/Cohabitation.  At the meeting the woman said 'technically I shouldn't leave Belgium for six months, though within the Schengen I wouldn't get checked so it should be okay'.  I have a train ticket and weekend planned in London and I was wondering, since its outside the Schengen zone is it going to be a risk to try to get back into Belgium. Can I just use my US passport to get back into Belgium?  Would the information that I have a conditional residency in Belgium and technically shouldn't leave come up when they check my passport in the control?

Thank you for your help...

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