I can't find a name for the episode so I'm going with S1E3.
Luther arrests his suspect for returning to the scene of the crime and attempting to disinfect it. However he has no cameras, no finger prints and no DNA...
How can he hope to convict the killer (he's already been told the case has to be watertight) simply by saying he found him there?
Luther finds the boat where Lucien Burgess commits his crimes and enters without exigent circumstances (from Wikipedia, "a situation where people are in imminent danger, evidence faces imminent destruction, or a suspect will escape"), meaning that any evidence he finds there will be inadmissible. He tells Lucien that he has discovered evidence tying him to the crimes, causing the killer to return to the boat - a location he would not have known had he been innocent - and simultaneously creating exigent circumstances.
Partial documentation here: http://www.avclub.com/review/luther-episode-three-46863
