If you want a digging to go unnoticed, you don't try to conceal the digging; you dig in a place where lots of other mundane diggings are already happening.

So, not going out in the middle of the night in ninja swit; but chearful gardening in the Church's front yard instead.

You can even put a heavy, thorny bush on top, so people will be too lazy to undig it by accident.