I don't think any monsters know of Beethro's presence after taking the invisibility potion until he steps in their smell radius. I think it would be a little inconsistent to have Brains slightly detecting Beethro if he is invisible.
I was thinking along the lines of the brain using a little logic. It's seen Beethro once, and he's disappeared again. Whether he's left the room, or if he's invisible, it makes sense to put the spiders where they are camouflaged.
As to the inconsistency, it would, yes, be the first sign that brains can be proactive, as well as reactive. But it is still a simple rule, the behaviour easily observed. It's not like I'm requesting that brains have the ability to set traps, amass forces before attacking, make coordinated attacks from multiple directions, defend itself, send roaches or tar babies scouting for an invisible Beethro, or anything like that
If Beethro is visible, but at that time inaccessible to the spiders, there'd be no such dilemna. Move the spiders so they're hidden. If a spider is not camoflaged, but there is an empty 'camouflage' square it can reach within a defined radius (3 squares?) move it towards that square.