Disclose detention without police record?

Andrea writes, "When I was 16 I was detained; I think the cops kept no record. But I'm still unsure of what to answer on part 3 question 1 (form I-821d).  Will the USCIS still see that incident, when they run a background check? Should I check yes or no?"

Generally speaking, if you are detained, a record will be kept.  If you are 100% confident that the police made no note of your conversation with the police, then it was probably not a detention.  Looks like you were not charged with a crime but there will still be a record in the local police's files.  It appears to me that you have nothing to fear, not for DACA, not for any other immigration benefit, but the best course of action would be to contact the police department and ask them for the documentation.  You can then truthfully answer what happened and attach the document, which will show that the police simply questioned you but you were neither fingerprinted nor photographed nor arrested nor charged with a crime.

In matters like this, the safest thing to do is to assume that the USCIS can find out anything and everything (it is not clear in you case what the cop did with the information because cops record everything and you just cannot be sure what will show up in your background search), because if you hide something, you are in a lot more trouble than honestly telling what you did.