As I explained to barbaratorrance there is no room for confusion here, but "polícia" can mean both the police force and a police officer: http://www.priberam.pt/DLPO/pol%C3%ADcia or http://aulete.uol.com.br/pol%C3%ADcia.
There is a subtle difference between the first definition from a Portuguese dictionary and the second from a Brazilian dictionary. It appears that when the word means police officer it can have both genders in Portugal but can only be masculine in Brazil. If correct, that implies "a polícia" can mean both "police' and "policewoman" in Portugal, but in Brazil "o polícia" must be used for both a male and a female police officer. (Wiktionary seems to agree with the EP version: https://en.wiktionary.org/wiki/pol%C3%ADcia).
Is "o polícia" currently used in Brazil or has it been entirely replaced with "o/a policial"?
This is true in Brazil, but as I tried to explain, things are different in Portugal and there "o polícia" means (the) police officer. See: http://cdp.portodigital.pt/Members/admin/profissoes_futuro_pdfs/justica-e-seguranca/policia.pdf
Yes, the sentence "The police find the bag" is a correct English sentence, but it's not a good translation here. "O policial" does not mean "The police", that needs "A polícia". "O policial" is "The policeman" or "The police officer" and in that case the correct translation of "acha" is "finds".
Based on other comments here, Duolingo mistakenly accepts "The police finds the bag" and I think that is also due to an incorrect translation of "O policial" rather than a poor use of "finds".
I wrote "the police officer thinks about the bag" here and it is not accepted. I understand that "achar" may mean "find" but in this case, why is it that when I put my mouse on "acha", it shows "thinks" as a second choice, which I think is inviting me to make a mistake... and how would I say that he thinks about the bag ? ele acha sobre a bolsa ?
If you can replace "think" with "find" then "achar" is a good fit: "I find religion is stupid" makes sense but "I find about religion" doesn't. Of course "find" can sound a bit stilted nowadays, but see definition 4 here: http://www.macmillandictionary.com/dictionary/british/find.