Q. Does one have to stand for an elderly person if he is unaware of it?
A. On question 1158, in regards to standing up for an elderly Talmid Hacham that is senile or blind and is not aware of the honor you are giving him, we wrote: “Ginas Verodim (Y.D. 4: 2),Birkey Yosef (Y,D, 244: 2), Rabi Akiva Eiger ztl (Y.D. 240: 7), Ben Ish Chai (Ki Tetze 15) et. al. rule regarding the blind, that one has to honor them even if they are unaware of the respect given. Poskim maintain that it is similar to embarrassing the blind which is prohibited (Bava Kama 86b). Derech Sicha (P. 374) asserts that the same applies to the elderly or ill, who are oblivious to the treatment given to them.
Horav Shlomo Millers Shli’ta opinion is similar.
On question 1159 in regards to standing for a Talmid Chacham or an elder who is unaware of the honor given to him, (he is facing another direction), if it is better to make him aware, we wrote: Horav Shlomo Millers Shlit’a opinion is that in general there is no need to make the recipient of the honor aware that you are standing for him or honoring him, unless he needs to benefit emotionally from that knowledge;( e.g. to rise his depressed spirit).
In principle, the Rov maintains one should rise slightly, as mentioned above, even when the elder is not aware of it
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a