Spying

> This leads me to ask, if spying is haram, then can a person be accused
> if someone obtained evidence through spying?

No. It is not valid to accuse a person of a crime through evidence taken
from spying. All such evidence taken from spying or unwarranted searches
and seizures is to be totally disregarded. The precedent for this was set in
the famous incident during the Khalifate of `Umar ibn al-Khattab who
overheard music and smelled alcohol from a residence of a man in Madinah.
`Umar then scaled the man's backyard wall and told him that he had committed
a felony (e.g., drunk alcohol). The man then defended himself by saying that
although he may have committed one crime, `Umar had committed three:
(1) spying by obtaining evidence from a private residence, (2) entering
through the back of the residence whereas the Qur'an commands us to enter
through the front [al-Qur'an 2:189], and (3) entering into a private residence
without prior permission. `Umar then acknowledged his mistake and did
*not* bring the man to court.

> Then I wonder, what is the definition of spying.

The definition of spying is obtaining evidence/information from an exclusive
private space of an individual. An exclusive private space may be a
closed house/residence/apartment/hotel-room/suite/room, private vehicle of
transportation with its doors closed, etc.

For example, one may not use instruments or one's eyes to listen
to or see what is going on inside a private hotel room.
Now if one senses danger for a third party (e.g., a kidnapped child), then the
authorities may use surreptitious methods to verify if the person in
danger actually is located within a private place to facilitate the
safe release of the victimized person.

References:
[Qawanin Fiqhiyyah: volume 1: page 368: line(s) 20-21: {book 21, chapter 5,
section 4, one may not spy to correct wrongs}]