notice — an announcement or intimation of something impending; warning: a day's notice.
spy — a person employed by a government to obtain secret information or intelligence about another, usually hostile, country, especially with reference to military or naval affairs.
catch sight of — to make out by means of the eyes; discern; see
sight — the power or faculty of seeing; perception of objects by use of the eyes; vision.
discover — to see, get knowledge of, learn of, find, or find out; gain sight or knowledge of (something previously unseen or unknown): to discover America; to discover electricity. Synonyms: detect, espy, descry, discern, ascertain, unearth, ferret out, notice.
spot — a rounded mark or stain made by foreign matter, as mud, blood, paint, ink, etc.; a blot or speck.
detect — To detect something means to find it or discover that it is present somewhere by using equipment or making an investigation.
find — to come upon by chance; meet with: He found a nickel in the street.
glimpse — a very brief, passing look, sight, or view.
view — an instance of seeing or beholding; visual inspection.
watch — to be alertly on the lookout, look attentively, or observe, as to see what comes, is done, or happens: to watch while an experiment is performed.