Re: after all the yrs of teckin, i've never been asked this.....
Usually time capsules are put into cornerstones of buildings. And if so, the problem becomes immediately apparent, that your machine would simply hear all the other metal in the sides of the building (rebar, wires, pipes, etc....) As most buildings are riddled with metal, in some form or fashion within the walls.
But if, as you say, it's underground "by a flagpole", then I would wonder if it might be buried directly beneath the flag-pole? In which case you'd have to ask yourself what the flag-pole is make of, at the base point where it enters the ground. Because if that's a metal flag pole (or even a concrete base, yet metal unseen inside the concrete block), then you won't be able to get within a foot of that, lest the flag base merely set your machine off.
All you can do is simply wander around 360* around the base of the flag-pole, and check (odds are, it wasn't buried far from the pole, d/t they naturally wanted a mental marker of where it was, but as I say, it could be utterly underneath it though, too).
If the area is littered with small cr*p (tabs, foil, nails, etc....) you can go with a 2-box unit. Those will only find soda can objects and larger. The perfect "discriminator" to only go after large objects, and not even hear the other stuff

But a 2-box detector is not going to be able to get even remotely close to a metal flag pole though, as it can probably detect such a monstorous metal item from several feet away (not sure how much to the SIDES a 2-box unit will start to pick up, verses directly underneath it, but I'm assuming that it too, just like a standard machine, can sense on the sides to a certain degree too).