What am I missing, because it strikes me as unlikely that when a random phone was turned in to LE, LE took the time to get past the PIN (how and why?) AND took the time to realize the phone was related to a crime, instead of just putting it into a box with all of the other "found random unclaimed items." For some reason I can't make sense of this detail.
This is just speculation, but I was thinking that perhaps LE had already predicted they might find an abandoned phone related to the two suspects specifically in Ayer.
Original articles like this one say that the dumped phone in Ayer led LE to the knowledge of another phone, and it was this other, previously unknown-to-LE phone that was able to be tracked to Abington: Phone found in Massachusetts triggered new Elijah Lewis search
So I think that LE was trying to track their movements via the phone they knew about, and already knew that it went off the radar, so to speak, in or near Ayer. Then when a phone turned up, they immediately knew there's a chance that this isn't just a random unclaimed item. But this is all just an idea based on what's been released so far, we may find out later the home screen has a photo of one of the two suspects, or didn't require a code to unlock, or something mundane like that. Luck was certainly involved in finding Elijah so quickly.