Or a perp was familiar enough with Papaw's habits and routine, and knew Papaw would most likely have his phone on his person, so they made sure to turn it off as soon as they had the opportunity to do so, and then:
1. Discard it at a location where it most likely wouldn't be found (e.g., a body of water), and they could do so either without being seen (e.g., taking a boat on Lake Tawakoni; at least two public access boat ramps within a 20-minute drive from the Chambers home; weekday; no precipitation with a high of 77F in Greenville on Mar. 10)
http://tpwd.texas.gov/fishboat/fish/recreational/lakes/tawakoni/access2.phtml#areab
https://www.google.com/maps/place/3...!4m5!3m4!1s0x0:0x0!8m2!3d32.90492!4d-95.99871
https://www.wunderground.com/histor....zip=75474&reqdb.magic=1&reqdb.wmo=99999&MR=1
3.Or at a location where if someone happened upon them, no one would be suspicious of them for being there (e.g., a popular hunting spot, a random gas station, where they could just throw it in the trash, etc.)
http://tpwd.texas.gov/regulations/outdoor-annual/regs/counties/hunt
Or Papaw was intending to harm himself and didn't want to be found quickly, so he shut it off either before he left the property or as soon he remembered to (even though I continue to believe that is not what happened, not only because I'm just not getting that vibe, but also based on some of the evidence discovered).
Regarding the phone, one thing I found curious -and makes me somewhat hopeful- is how the Sheriff reacted to reporters' questions about it, and his answers at the press conference (starting around the 10:05 mark in the video at the link; second video from the bottom: "Raw Video: News Conference on Missing Fighter"):
A reporter asks "The ping of the phone didn't get you anything?"
Sheriff answers "No sir, no sir."
A different reporter asks "Did it ping at all, or ...?"
Sheriff: "It did, it did. I'm not, I'm not allowed to elaborate on that 'cos it's part of our investigation, but it did ... a couple of places, but [lets out a sigh] we followed up on those, and nothing."
http://www.nbcdfw.com/news/local/My...-Community-Investigators-416170523.html?t=604
Hopefully, it did get them something and they are keeping it close to the vest.
I'm assuming that the "Find My iPhone" feature, as well as all apps with GPS access on Papaw's phone were disabled at the time of his disappearance.