Well, it depends on whether you want Alkalus to make a clean escape. If so, then have the party stumble upon him when least expected, giving them a slight chance of pulling together and realizing what's going on before he can run. If not, then have the party realize that he's nearby and let them plan in infiltration or assault, putting our friend Alkalus between a rock and a hard place, if possible.

It would also depend greatly on his emphasis and build as a character. Is this the BBEG that manipulates everything to work for his advantage, keeping his hands clean and watching everything play out, or does he take things into his own hands, sneaking into facilities and crushing those in his path? That's a big aspect of BBEG decision-making.