standard procedure?

I have a couple of special agents with the Secret Service investigating a month old crime scene on public property in a park, looking for potential stuff the locals would not have known to look for. The agents are also in a secret division investigating potential threats to national security. They carry ID, but not always weapons, and dress in plain clothes, not suits. Seeing as the stuff they investigate is classified, they don't generally announce to the local cops who they are or what they are doing unless it's something for which they expect to have break out the firearms or commandier local equipment or manpower. It's a need to know basis, and usually the locals don't need to know.

Does it seem reasonable that they try to remain secret even from the local cops?

I don't think I'm taking it to an extreme, but thanks for the replies. It sounds like I'm on a believable track.