You could try pouring a bucketful of water onto the pavement, all around the cover, a foot or two away and seeing if any naturally flows towards the hole. That will tell you what is happening when it rains, or when people wash the car.
If you're on flat ground to the extent that your neighbour's meter is roughly on the same elevation, or they are literally only a few feet apart on the same piece of pavement, such that the drainage situation is practically identical, then it could be a leak.