@AppleQer, most of the Apple Stores I've visited are very open and have huge windows with the outside view so the reception is generally good. Phones on display work on wifi and GPS, but no cellular I think (none of them seem to be connected to a network at least). While I agree that indoor testing like this is not ideal, this is the only way to test store phones and is consistent with my personal phone testing outside it. Even in much worse indoor conditions there is a very distinct behaviour only the faulty phones exhibit and there's no doubt when comparing it to a properly working phone.
Find an iPhone 6S (or SE) nearby an iPhone 8 or X (all of them store phones just to be fair), take two of them in your hands and if the 8/X is faulty you'll see a very stark and consistent difference in their behaviour. Repeat with other phones and soon you see a pattern of the "faulty" phones (if there are any). You are not looking for extreme precision here (although most phones were surprisingly precise in my experience), you are just waiting for the start of very typical, extreme jumps (often 50+ meters), smooth drifts like someone is flying around (sometimes very quickly, sometimes slowly but eventually drifting too far away), and larger than normal, pulsating circles around the location (they will often shrink and expand extremely, very quickly). As long as you hold the phones this behaviour does not stop really. Waiting 10+ minutes does nothing to help such phones. In fact it often makes it worse. Soon you learn recognize the pattern and don't need a lot to confirm the existence of this issue. You need a lot more time to confirm the absence of it though.
"Working" phones never exhibit such behaviour even if they cannot get a precise lock. There are also slight nuances to the smoothness of the jump. Location on such phones will jump around with much less gliding/smoothness, settle down eventually, and will continue to slightly shift around but it's going to be like little instantaneous jumps and well within tolerance of say 5-10 meters. Also, the circle(s) around the location remain in roughly the same size even when pulsating. They change but rarely so extremely. Very sheltered buildings, rooms, tunnels etc. are obviously an exception if it needs mentioning.
But, like I said, in some stores I've been able to reproduce this with ease on many phones. In two stores (only recently) I was unable to trigger this behaviour not even once (one of them was a reseller with terrible indoor conditions). With my new iPhone X, I am unable to reproduce this in any of the previously tested areas. I got to know the behaviour quite well, looking at it every day on my 8+ and comparing it to my wife's 6S in various indoor/outdoor conditions, so now the X is behaving very much like the 6S in each of those conditions.
Hope this helps
P.S. Almost forgot. If you have a "faulty" phone yourself, compare your personal phone to and 8/X in the store. Either both of them exhibit the same issue, or alternatively, if the store phone is "good" it behaves better without cellular than yours does with everything ON. Either way, not a happy outcome for your phone.
