They weren't aware of Robin's experiments involving Hiruko, it was the other staff at the facility who discovered his victim in that room. Maru's objective at the time was just to save Kiruko from him, which he did. I don't see how they'd know where he ran off to or why tracking him down would be their priority when they're not aware of what he was up to (until this chapter of course). He will meet them again for sure though.
You are totally right, all they did was allow a rapist to escape after they had cornered him, immediately after he was caught fucking one of them.
Why would they not be concerned with him being free? Is not like they knew the guy was up to no good, he is just a rapist, why would they letting him go be bad? and now they know that he is just a murderer, and that he is just doing human experiments, they can deal with that later
Actually it is not even their problem really, Robin hasn't actually done anything to them personally, so they shouldn't even have to be worrying about him, they don't have to deal with him now nor later
Plus, metanarrativelly speaking, eventually they maybe will cross paths with him anyway, right? maybe they will share some tea and cookies and reminisce about the good ol days, and if they don't meet again, then that's just life
But now they have more important things to do, because Hiruko is on the clock and she is a certified professional handyman that gets stuff done
That stuff with Robin is just whatever really, a detour at best, a filler story at worst, is like a fetch quest from a random NPC, meeting Maru's twin in heaven, now that's the main deal, and what they should be focusing on
I agree completely now, I shall forget about it, and stop worrying too