Quote from perpetual
Maybe baby Beel just doesn't want anyone stealing Oga away from him o.0...
I mean, Mikki was-sorry, IS still Oga's friend (i guess) and baby Beel just doesn't want anyone near him. Anyways, Mikki gives off some random aura of gayness and baby Beel obviously dislikes that.
Baby Bel is a powerful demon, so he should have a 6 sense regarding the stuff hidden in the closet.