In Fate/stay night: Unlimited Blade Works, when Rin had just summoned her servant (Archer) and walked into the room he was in, did he notice Rin straight away as a girl he was in love with or knew? And if so, why didn't he tell her?
Did he notice Rin straight away as a girl he was in love with or knew? No, he did not. It is mentioned here:
'Archer was fairly close to the Rin from his timeline, though he did not recognize the Rin who summoned him as a Servant until she revealed her name.' This was because Rin messed up the summoning but he regained his memories after hearing her name, though he pretended not to.
Why didn't he tell her?
Because his goal at that time was to kill Shirou, not to be with the Rin in his timeline. He has nothing to gain or lose by revealing who he was since his only wish at that time was 'to die' and erase his existence.