He prayed in a place, then after the prayer he discovered that there was a cross near that place
Question

                If someone prays in a place, then discovers later on that there was a cross near that place, is his prayer accepted?
Praise be to Allah.
    The cross is a symbol of the Christians 
    which they venerate because they believe that the Messiah was killed and 
    crucified on it. This is a false belief that was declared false by the Holy 
    Qur’an, which stated that it is a lie and misguidance. Allah, may He be 
    exalted, says (interpretation of the meaning):

    “And because of their saying (in 
    boast), ‘We killed Messiah ‘Eesa (Jesus), son of Maryam (Mary), the 
    Messenger of Allah,’ - but they killed him not, nor crucified him, but the 
    resemblance of ‘Eesa (Jesus) was put over another man (and they killed that 
    man), and those who differ therein are full of doubts. They have no 
    (certain) knowledge, they follow nothing but conjecture. For surely; they 
    killed him not (i.e., ‘Eesa (Jesus), son of Maryam (Mary) )”

    [an-Nisa’ 4:157]. 

    It was the practice of the Prophet 
    (blessings and peace of Allah be upon him) – as it says in Saheeh al-Bukhaari 
    (5952), in a report from ‘Aa’ishah (may Allah be pleased with her) – that he 
    did not leave anything in his house on which there were crosses, but he 
    would break it and change it. 

    Based on that, it is not permissible for 
    the Muslims to have crosses on anything in their houses, furnishings or 
    mosques. 

    It says in Fataawa al-Lajnah ad-Daa’imah, 
    in fatwa no. 7810: 

    The cross is a symbol of the Christians 
    who put it in their places of worship and venerate it; they regard it as a 
    symbol of their false belief, which is the crucifixion of the Messiah ‘Eesa 
    ibn Maryam (Jesus son of Mary – peace be upon him). Allah, may He be 
    exalted, stated that the Jews and Christians were lying about that, as He, 
    may He be glorified and exalted, says (interpretation of the meaning): 
    “but they killed him not, nor crucified him” [an-Nisa’ 4:157]. So it is 
    not permissible for Muslims to put crosses on the furnishings in their 
    mosques or elsewhere, or to leave them there; rather they must get rid of 
    them blotting them out and erasing their features, so as to avoid evil and 
    avoid resembling the Christians in general, and with regard to that which 
    they regard as holy in particular. End quote. 

    But if a person prays facing towards 
    something on which there is the design of a cross, or in a place where there 
    is a cross, his prayer is still valid, even though it is makrooh, and he has 
    to avoid that in the future. 

    The scholars of the Standing Committee for 
    Issuing Fatwas were asked: Can a Muslim pray on a mat on which there is the 
    design of a cross? 

    They replied:

    If the question is about the ruling on a 
    prayer that was done on a mat on which there is a cross, the prayer is 
    valid, in sha Allah, although it is makrooh. 

    However, if the question is about the 
    ruling on doing that in the future, then he must remove the cross from the 
    mat, by erasing in in such a way that will remove its features, or by 
    placing a fixed cloth over it, or by replacing this mat with one on which 
    there is no cross, because it is narrated in a saheeh report from ‘Aa’ishah 
    (may Allah be pleased with her) that the Prophet (blessings and peace of 
    Allah be upon him) would not leave anything on which there were crosses but 
    he would cut it out.  

    End quote from Fataawa al-Lajnah ad-Daa’imah 
    (3309) 

    The scholars of the Committee were also 
    asked: 

    What do you say about watches on which 
    there is a cross? Is it permissible for us to pray wearing them or not? 

    They replied: 

    It is not permissible to wear the watches 
    on which there is a cross, either when praying or otherwise, unless the 
    cross is removed by scratching it off or covering it up. But if a person 
    prays wearing it, his prayer is still valid. 

    What he must do is hasten to remove the 
    cross, because it is one of the symbols of the Christians, and it is not 
    permissible for a Muslim to resemble them.

    End quote from Fataawa al-Lajnah ad-Daa’imah, 
    question no 9 in fatwa no. 2615 

    The ruling mentioned above, that it is 
    makrooh, only applies to the one who is aware of its presence. If it was the 
    case that he was not aware of the cross being there until after he had 
    finished his prayer, then there is no sin on him and it was not makrooh in 
    his case, and his prayer is valid, in sha Allah. 

    And Allah knows best.