Do you realize when you Survey a tract there are more parties involved than just your client's property. You have the adjoining properties, that you need to check to make sure they have the land called for in their deed, if they don't you need to figure out why and be able to back it up, same with your clients land if they have more or less than they should. Also you have to make sure any public roads, alley ways etc.. have the land they call for, then you must meet State laws, rules, and regulations. Right now you are not surveying the adjoining property, and seems to me you are to lazy to do the research to even find out who owns the land. If it is State land I am not sure if Montana allows Adverse Possession against the State, if is it public land you can't.
How big of a Cap are we taking about here, is it a 1 foot gap or big enough to drive a car down to access property somewhere? It may have been the developers plan to leave the gap for access to another tract, or to come back in a few years and sell them the land so they can have river front property. Since the subdivision plat does not go to the River, there is a reason find it.
Also you really need to find out who owns the tract, is it the State, Public, heirs, was it sold last year? Get a title company involved.
You have the same Ethics as a lawyer, how much are you charging to steal land from someone?