I had this happen once with our Toro system. I replaced the solenoid and that fixed it. I easily found the solenoid at Lowe's.

Get the manufacturer's name off the HYDRO-INDEXING VALVE (AKA hydro-indexer) and call your local irrigation wholesaler to see if they have a 3-zone cam. THEN make sure that the numbers on the top of the cam line up with the active outlets. If one lines up with the plugged outlet, you're in for a real treat - repiping the indexer.

lafutura, you have piqued my curiosity as to why you would want to install a perimeter drip line on a lawn that is already watered by a system. I would think that installing a gate valve or similar ahead of the drip zone would allow you to regulate the water flow into the new drip line. Concerning the other question, there ought to be a manifold on the premises. Either all the piping to the zones start out at a central location and are routed their separate ways or else all the valves (and piping) are in a central location. Another possibility is that a main 'hot line' was run from the water source and the zone lines (and valves) were tapped from it. A radical approach would be to disconnect the wire from the unidentified terminals (one at a time) at the clock and observe which area wilts. This is recommended only if you can critically observe your whole landscape daily. You want to spot the earliest symptom of wilt to avoid serious damage to you landscape.

I have two systems - the rainmachine (8) at my rental and the orbit bhyve (I think it's 12) at my house. Because I've been dealing with reseeding off and on for a couple of years (note, fix any broken sprinkler heads first), I've had my rental/RainMachine on more me-controlled rather than weather based. Remote access is awesome, I definitely have to say, since that's allowed me to tweak the schedule based on the rain predictions (since seeding requires more than in maintenance state, and I guess a ding would be that there's no "seeding" choice for the plant type, but doubt anyone has that). I chose the rainmachine about 2 years ago based on the fact that I wanted a controller that didn't rely on the company being an ongoing entity - one of these companies WILL eventually go belly up. It's been working great for me in this scenario, though I do currently have an issue with remote access on it, but I think this has more to do with my tenant's internet connection and I haven't had the time to troubleshoot it with the holidays keeping me busy. I literally have not had to open the weatherproof box I store it in for the last two years, so much so that I've forgotten how to use the buttons on the device (of which there are really none, being the 8 version). For the B-Hyve, I've had it for about a year and a half, and have it using SmartWatering (weather based). I did purchase a sensor for it at one time, as that was required in order to get the Water District rebate (so this one essentially cost me $0). I will give it much credit because all my plants and grass are still alive relying on it to make the watering decisions. I have definitely seen it skip in periods of rain (we are in Northern Cal, in a perpetual drought, but saw record breaking rain last season). The app can be a bit quirky - like it will have watered that morning, but the rest of the parts of the app need to be caught up on this info like saying tomorrow it will smart water, but wont end up actually watering again tomorrow because it does eventually catch up. It's gained my trust (to water as well as not water), so I've been able to look past that. I am currently having an issue where my front yard zones are not turning on, neither on the app or directly at the controller, but I'm guessing that this is also something unrelated to the system (like I probably need to check the wires) and again, havent had time to troubleshoot it (not urgent since it's almost winter), I will say that the b-hyve having knobs is nice, as I know the gardeners could probably test the system if need be, while with the rainmachine - the tenant's gardeners doubtfully have loaded the app on their smartphones! And I guess the b-hyve gets points because it comes in an outdoor enclosure (had to make my own for the rainmachine, since I needed it to be outside of the tenant's dwelling). And since it's orbit, you know they are not going anywhere anytime soon. A ding with B-hyve is that I don't find much online/in forums about it for troubleshooting - not sure why orbit wouldnt sponsor that so that people can get the wisdom of the crowd. I say with either of these two, you probably cant go wrong. The rainmachine is pretty, so if you plan to have this in your house, I'd chose that. I guess I would say I like the graphs of the rainmachine app better, but that may be unimportant once you just have it at set-it-and-forget-it state.