Although being SWR suburban, the chance of a guard actually walking through the train, at least once during a journey, rather than hiding in back cab is fairly low.
I can't say guards never ride in the back cab, but in my expereince guards on 701s ride in the passenger ara and dispatch from one of the doors. I spoke to one this morning (and so did several others, to seek clarification as the announcements and screens were contradicting his own announcements)
You will rarely see guards in the back cab of a 701/0 simply because it's very inefficient to be there. Guards will usually only position themselves in the back cab if the train is very busy/full and standing during peak periods or events. What is common on much every station with curves and thus CCTV dispatch monitors is that the nearest monitor to the rear cab of the train is two or three coaches down. If a guard is in the back cab, they are to walk down and check the monitor/signal if there is one, walk back to press the close doors button, walk back again to the CCTV monitor to check all the doors have closed correctly and that it is safe to dispatch the train, and then walk back to the rear cab. You can be sure this will add a couple of minutes to most station stops, so therefore the vast majority will avoid being in the back cab for extended periods, unless they have to for a full and standing peak service, because you pick up a lot of delay minutes unless dispatchers are available on curved platforms like they are during peak periods.
If the platform is straight then the guard can be in the back cab and dispatch safely without having to move further along the platform. But the opportunities to do that for more than one station in a row are few and far between. There are a couple of routes/service patterns where it is possible for the guard to sit in the back cab for around 10-15 minutes. But 10-15 minutes on a journey that is probably around an hour long is not much at all. So the vast majority of the time the guard will be in then passenger areas visible as you would otherwise lose too much time. Are some guards more proactive than others in walking up and down the train? Yes. But sometimes it is not practical. This is why I have tagged Snow1964's comment. Often when the service is particularly busy or when the guard is familiar with particular services and how busy they may become, it can be much better to stay in a particular coach where they can dispatch from most platforms without having to move their local door. You will find many guards in either coach 5 or 6 as these coaches line up well with nearly all CCTV monitors. At the end of the day the safe operation of the train is the most important and being visible/walking up and down the train takes lesser priority, and anything that helps operation is key. There are many other legitimate scenarios where it's better from the guard's point of view to stay in a particular coach for a large portion of a journey, but I won't go into them now.
What many forget is there is no cab/guard's office/refuge at all on a 701/0 except for the back cab - in coach 10 of 10. That's a long, long way to go to get an item from their work bag, to have a sip of water etc. If you see a guard in the back cab it's probably because they need a small moment to catch their breath or to fetch something.
I understand many have a dim view when they see a guard staying in the cab for nearly the whole journey when the train isn't very busy, but in my view that is mostly not true for 701 services. Specifically the 701/0s. However, when the 701/5s come into service soon and they pair two units together, you will naturally get a bunch hiding in the cab in the middle for the whole journey (as seen on SWR's other stock) since being in the right/convenient dispatch positions will be a non-issue. Of course, it must be said that there are many valid reasons why a guard must be in the cab, some of which are mandatory according to the rulebook, but again, that's for another time.
