I agree it's strange. If you're referring to the grandfather speaking about Libby's phone "pinging around town" and "5-6 hours ago"... I am not 100% certain but I would imagine you don't need a warrant to use pings to locate a missing person. I'm thinking LE could have done it that evening, especially in a town of 3,000 people, and confirmed the phones were on around 3-4pm.
I'm guessing LE briefed the families, and let them know the ping information as evidence of them still being in the area. That may be why the families suspected no foul play, the search was called off at dark, etc.
I'd be curious about the cell infrastructure within Delphi. I think you need 2+ base stations to reliably triangulate a location, otherwise you're just able to identify some wide radius the phone was within, probably an entire town). Not sure about this though. IIRC in the February 13 interview with the grandfather he mentions "we have one here", referring to the base station / tower within Delphi. If they only have one (which seems likely for a small town), they wouldn't be able to assess much.