Andy, I wouldn't be detecting a College Campus when Classes are going on. I only hunt them when they are out for the summer and no Classes are scheduled so you have the Campus to yourself. This way you don't have to look behind your back on every dig and you can relax and focus on what your doing so when done doing the Recovery, the Plug and the area around the Plug looks good as new.
Now as far as how Deep one digs in well maintained Parks, that will depend on the user's ability in retrieving Targets and how well they clean up after each recovery. If I get Deep Targets in the 10" Range, an the Park is well maintained, I still dig until I get that Target out of the ground. I try to cut only about a 4" Plug and usually when I flip it open, I take my Left Foot and hold the Plug Open while I am kneeling down doing the recovery. Doing this allows me to somewhat hide the sight of the Plug and the only way to see what I am doing would to be to walk up directly in front of me since I use my Detector to block one side of the hole and my body to try and block as much visual of the recovery as possible. I remove the dirt and set it in a pile near the hole until I find the Target. Then I will do a quick check in the hole with the X-1 Probe to make sure there's not other Targets and then push the dirt into the hole. If all the dirt won't go back in, I will put the plug back into the hole and take my Lesche Digger and fan the grass with the serrated edge until the dirt has blended back under the surrounding grass. Doing this acts like a small rake and if done right, there won't be any signs of dirt or the Plug itself. I make sure to firmly press the Plug into the ground and then brush the Plug with my foot to get the grass to stand back up.
Now if the Target starts getting too Deep, say 12" or so, then you call it quits since the Target will most likely be something rather large and not a Coin. The only way I will keep going is if the Target didn't have a good Signal say at 8" and it was real weak when you swung the Coil over the Target before the retrieval. If the Target was Loud and had a Strong Signal, say at 6" and now your down close to 11"+, then it most likely is a large piece of metal and not a Coin.
Also, if the Target starts to Null, say at 8", I still dig a bit more dirt out to see if the Target was just out of reach for the X-1 Probe to get a Signal from and if it still Nulls, then I go into Pinpoint and find the location of the Target and try and remove whatever the Iron Object is. If found and removed, then I re-check the hole to make sure there isn't a Coin that was with the Iron. If it seems that the Iron Object is too hard to locate or is Deeper, then I will just cover the hole and try and make a mental note of that Target and the area where it is in case of a return trip to that spot later on.
You sure are right about being nervous when people walk by and see what you are doing Andy. If I find a Target that sounds good and is Deep on the Depth Meter, then I will look around to see where the people are and then I will put my back to them and then do the recovery since this will buy you some extra time before anyone notices what you are doing. Something to think about using when there are people present, that and waiting until they walk by or leave the area, then go back and recover. I have done this also by either marking the area with a Twig or some other way of knowing where the Targets are so you can retrieve them later on.
Well, I hope this makes sense Andy and hope it wasn't too long since I try to explain exactly how I do things when out hunting and retrieving. Good Luck and HH to you on your next outing Andy.