Word for observing others

He’ll have a job that involves very little social interaction, but it puts him in a position where he can observe others unnoticed.

Его работа почти не требует общения, но ставит его в такое положение, что он может наблюдать за другими и оставаться незамеченным.

No, no. I learned it from observing other marriages.

Я узнал это, наблюдая за другими браками.

If you know synonyms for Observing, then you can share it or put your rating in listed similar words.

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And for a long period of time.

Example:

He spent several nights […] the terrorist’s building.

I checked the synonyms of watch (noun lookout), but found nothing that could fit. There’s also track, but I think it implies walking:

track
follow the trail or movements of (someone or something), typically in
order to find them or note their course.

Maybe to keep vigil?

If informal usage would work for you you could use:

stake out as a verb:

they’d staked out Culley’s flat for a day (ODO)

He spent several nights staking out the terrorist’s building.

The act of checking out a location, particularly in the context of the military, is called reconnoitering. One can also make a reconnaisance of a place, like before attempting to occupy it. (M-W)

To «monitor» is to continuously observe a defined subject

To «monitor» has for me the strongest implication of continuous attention regarding new knowledge to gain.
In the example, it is about collecting all relevant information, not to work with the infromation instead, which would draw attention from collecting.

So studying it, and investigating — all that comes later, based on the result of monitoring.

Scouting was what came to mind immediately for me.

He spent several nights scouting the terrorist’s building.

answered May 13, 2015 at 16:32

Sizing up sounds best to me although slightly colloquial:

He spent several nights sizing up the terrorist’s building.

or more formally evaluating at the risk of losing some of the flavour:

He spent several nights evaluating the terrorist’s building.

answered May 13, 2015 at 16:17

Combing through is an expression that fits your example well.

Auditing, like auditing a college course

«peruse» — IMHO the best word possible to mean, “observing something with the purpose of gaining knowledge from it”. Although «examine» is pretty near exactly the same.

The most reliable knowledge is that acquired by the scientific method.

Four essential elements of the scientific method are iterations,
recursions, inter-leavings, or orderings of the following:

• Characterizations (observations, definitions, and measurements of the subject of inquiry)
• Hypotheses (theoretical, hypothetical explanations of observations and measurements of the subject)
• Predictions (reasoning including logical deduction from the hypothesis or theory)
• Experiments (tests of all of the above)
  _{Wikipedia: Elements of the Scientific Method}

So to gain knowledge scientists need to characterize the subject of inquiry. One possible term for this process might be «data collection» but you’ll be hard pressed to find a better term than your own, as @RegDwigнt already commented

observe

transitive verb

• 1.a. To be or become aware of, especially through careful and directed attention; notice: observed a car leaving the property.
• 1.b. To watch attentively: observe a child’s behavior.
• 1.c. To make a systematic or scientific observation of: observe the orbit of a comet.
  _{TheFreeDictionary}

A great example of making observations over a long period of time was

Tycho Brahe, born Tyge Ottesen Brahe (14 December 1546 – 24 October 1601), was a Danish nobleman known for his accurate and
comprehensive astronomical and planetary observations. He was born in
Scania, then part of Denmark, now part of modern-day Sweden. Tycho was
well known in his lifetime as an astronomer, astrologer and alchemist,
and has been described more recently as «the first competent mind in
modern astronomy to feel ardently the passion for exact empirical
facts.«
_Wikipedia

answered May 13, 2015 at 18:05