I don't buy it. If it's really there, then you don't hide it from view. I can't believe something exists if only one person may see it and all others are kept away by scary stories of what will happen to them.
There are people that believe that the Ark has been located, but its existence has not been revealed because it would lead to an immediate war between Israel and nearly all Arab nations. The way it is posed, a Jewish Israelis would proceed to rebuild the Temple, and that may include tearing down the Al-Aqusa Mosque, which would certainly result in an all out war.
I never gave it much thought that it was actually known, because like you, I thought if it was, it would certainly be made known to the public. But this idea that the Ark would be the catalyst to rebuilding the Temple made me think that it may be kept hidden for now.
If you get the chance I would highly recommend Hancock's book, it is really a good read.
Go to the Old Testament, my friend, and read how often the ark was seen once it was in the sanctum sanctorum. Once a year, and then only by the high priest.