Legal bounds?
The law doesn't matter in the grand scheme of things.
Would it work?
Its better than doing nothing.
Would it help?
Its better than doing nothing.
This thing you're describing is, at its core, a basic community defense thing. Everybody has a few phone numbers they can call/text with "news" when needed, no one person has everybody's number and no one person is responsible for contacting everybody. Letting people in the area know there are "strangers behaving strangely" or "hey, do you know what's going on with all the cops going door to door?" is not outright a crime.
Its not your job to know the immigration status of everybody around you or their family or to remember any conversation you may/may not have had with a person.