Time of occurence of janaba unknown.
Question

                If a man sees the signs of sexual discharge impurity (janaba) on his clothes, and he is not sure when the discharge occurred, and several prayers have passed, what should he do?
Praise be to Allah.If a man sees the signs of sexual discharge impurity (janaba) 
  on his clothes, and he is not sure when the discharge occurred, and he had performed 
  several prayers (salaat) not knowing, he has to perform major ablution (ghusl) 
  and repeat the salaat from the most recent sleep in which he was wearing this 
  clothes. However, if he finds out that the janaba was from a previous sleep, 
  then he should repeat salaat from the last sleep he thinks the janaba occurred1. 
  The references (adillah) on the religious duty of performing ghusl due to janaba 
  in preparation for salaat are many, one of which is Allah's command in the Qur'an, 
  a translated meaning of which is: "O' You who believe, do not get approach 
  salaat while you are drunk until you know what you are saying, nor while impure 
  due to sexual discharge (junob) unless you are traveling about until you perform 
  ghusl" (4:43). And Ali's narration (hadith), (may Allah be pleased with 
  him), in which he said "I was a man with frequent urethral discharge, so 
  I kept performing ghusl until the skin of my back started cracking, so I mentioned 
  this to the Prophet  peace be upon him (or it was 
  mentioned to him) so the messenger of Allah  (peace 
  be upon him) said, "Don't do ghusl if you see urethral discharge, just 
  wash your male organ and perform wudu' same as wudu' of salaat. If semen appears, 
  then perform ghusl"2. 
  This hadith indicates that appearance of semen requires ghusl whereas urethral 
  discharge only needs washing of the sexual organ followed by wudu'.From the book What Should You Do in the Following 
  Situtations...?
1 Al-Mughni ma'a al-Sharh il-Kabeer 
  1/199.
  2 Narrated by Abu Dawood number 206 and verified sahih 
  by Albani in Arwaa' al-Ghaleel number 125.