I think it's just a presumption that there probably should be more than just one piece of evidence. Technically, the police probably could search for just one, but would that be enough? So if it's such a general sentence, we'll rather use the plural.
"Dowody" is either Nominative (To są dowody = These are pieces of evidence = This is evidence), or Accusative (Widzę dowody = I see pieces of evidence = I see evidence). "Dowodów" is Genitive, and this what is needed here, after 'szukać'.
I do not know what original question was, but, here is the result of my research. Policja is Nominative singular it has no plural, szuka is a verb in gen. that the direct object of the verb is always gen. so, dowod becomes dowodow (accent over O should be there) which is plural in gen. The police are looking for all evidence not just 1. RPS