[quote=@Eklispe]
So because I'm not entirely sure of this. Servants have some sort of servant sense thing so they can vaguely sense the presence of other servants that aren't in spirit form/actively hiding? And some sort of similair thing for masters not hiding themselves?
[/quote]

Command Seals react in the presence of each other (i.e. Shirou grabs his hand in pain when Rin shows up) and Servants can detect other Servants (i.e. Lancer detecting spirit form Archer, Saber detecting Archer when he's going to Shirou's house with Rin, Iskandar detecting Gil, etc etc.)