Let's talk about our Red Rush strategy...

And if red sees it, what changes in your logic?

Pete's suggestion is aimed at preventing accidental red rushes, where someone is moving along and suddenly initiates a red rush.

If red doesn't see it, such a move from blue won't work because the attack from red won't be considered a red rush.

But if red does see it, then this move makes sense. We're talking about a situation where the map has already been explored enough, and blue knows that red sees it. And blue also sees red's actions.
If blue is so afraid of being caught in a red rush, then it's enough to not spend the night in a portal.