What is the meaning of the Prophet’s words, “I can see you behind my back”?

One of the miracles of which the Prophet (blessings and peace of Allah be
upon him) told his Companions is that he could see them behind his back, as
he could see them in front of him. 

Al-Bukhaari (741) and Muslim (424) narrated from Abu Hurayrah (may Allah be
pleased with him) that the Messenger of Allah (blessings and peace of Allah
be upon him) said: “Do you think that I face the qiblah (and do not know
what you are doing)? By Allah, your bowing and prostrating are not hidden
from me; I can see you behind my back.”

 Ahmad (9504) narrated that Abu Hurayrah (may Allah be pleased with him)
said: The Messenger of Allah (blessings and peace of Allah be upon him) led
us in praying Zuhr, and in the last row there was a man who prayed badly.
When he said the salaam, the Messenger of Allah (blessings and peace of
Allah be upon him) called him and said: “O So and so, do you not fear Allah?
Do you not see how you pray. You think that you could hide from me some of
what you do, but by Allah I can see behind me as well as I can see in front
of me.”

Classed as saheeh by Shaykh al-Albaani (may Allah have mercy on him) in
Mishkaat al-Masaabeeh (no. 811) 

The word seeing mentioned in the hadeeths is to be understood as it appears
to mean, i.e., that he could see them visually. This is indicated by the
report narrated by Muslim (423) from the hadeeth of Abu Hurayrah (may Allah
be pleased with him) who said: The Messenger of Allah (blessings and peace
of Allah be upon him) prayed one day, then he finished and said: “O So and
so, why don’t you pray well? Why doesn’t the worshipper look at how he is
praying when he prays? He is only praying for himself. By Allah, I can see
behind me as well as I can see in front of me.” 

An-Nawawi (may Allah have mercy on him) said, commenting on this hadeeth:
The scholars said: What this means is that Allah, may He be exalted, created
for the Prophet (blessings and peace of Allah be upon him) the ability in
the back of his head to see what was behind him, and he (blessings and peace
of Allah be upon him) was granted more extraordinary abilities than this.
This is not contrary to reason or textual evidence; rather the texts confirm
this, so we must believe in it. Al-Qaadi said: Ahmad ibn Hanbal (may Allah
have mercy on him) and the majority of the scholars said that this seeing
was in a literal and real sense.

End quote from Sharh Muslim by an-Nawawi (4/149) 

Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: There was a
difference of scholarly opinion as to the meaning of this. It was said that
what was meant is knowledge, either by means of Revelation so that he knew
thereby how people really performed the prayer, or by means of inspiration.
But this is subject to further discussion, because if what is meant is mere
knowledge, he would not have specified by saying “behind my back.” And it
was suggested that what is meant is that he could see whoever was on his
right and on his left, of those on whom his gaze fell if he turned slightly
sometimes, and he described those who were standing there as being behind
his back. But this appears to be stretching the meaning and it is turning
away from the apparent meaning when there is no need to do so. 

The correct view is that it is to be interpreted as it appears to mean, and
that this “seeing” was in a real and literal sense that was unique to the
Prophet (blessings and peace of Allah be upon him) and was an extraordinary
feat. Based on that, the author narrated this hadeeth under the heading of
the signs of Prophethood; it was also narrated under a similar heading from
Imam Ahmad and others.

End quote from Fath al-Baari by Ibn Hajar (1/514) 

Does this miracle include times of prayer and other situations? The text
refers to prayer; as for other situations, there is no text that speaks of
them. According to some scholars that is possible, but according to others
it applied only to the case of prayer. 

Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: The apparent meaning
of the hadeeth is that this was something limited to the case of prayer, but
it may be that this happened in all situations. That was also narrated from
Mujaahid.

End quote from Fath al-Baari by Ibn Hajar (1/515) 

Mullah ‘Ali al-Qaari said: The words “I can see you behind my back” do not
necessarily mean that this occurred all the time, because it is contrary to
the report which says “I do not know what is behind my wall.” Hence this was
unique to the case of prayer and his knowledge about the worshippers behind
him. And Allah knows best. 

End quote from Mirqaat al-Mafaateeh Sharh Mishkaat al-Masaabeeh
(4/197) 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: He could see them
behind his back, and this is something that was unique to the Prophet
(blessings and peace of Allah be upon him): in this particular situation
(i.e., prayer) he could see the people behind his back. But in other
situations, he could not see anything behind his back. 

End quote from Sharh Riyaadh as-Saaliheen (5/113) 

And Allah knows best.