[He frowned and let out a huff of air.]

Then if another does come, this venerable one suggests you stay still and allow them to pass. There is only so long in a day that this venerable one can make us both invisible. Using it to attack unseen is a waste with the manner in which these creatures fight.

[He is not so much assuming that Eustace knew they were invisible as he is letting him know that the option exists... and some of it had been wasted already. Though it certainly came with the implication that Eustace should have figured that out on his own.]