In Settlers of Catan, what are ways to "hide" your strength until the end?
Hoarding developments cards is a great way. Here's a few specifics on that, along with a few ways that don't involve development cards (note that you usually don't want to exploit your hidden strength until you think it will propel you to victory):

Hold off using a Monopoly card even if you get it very early in the
game. Wait for exactly that right moment when there are many of one
resource that you can monopolize and efficiently convert into several
settlements/cities/dev cards.
Hold off using a Road Building card until it results in you getting
longest road and a new settlement all on the same turn.
Hold off using a Year of Plenty card until you happen to have a card
combo in your hand where employing it results in a 3 victory point
turn.
Have a couple roads that go off to horribly unproductive locations -
but are nonetheless potential settlement sites. Many people
automatically assume you'll never build there - but you will on the
turn it vaults you to victory.
People often have no clue that you'll have longest road with
just 2-3 more road links closing 1 or 2 loops. Do this on purpose,
particularly if you have a hidden road building development card.
If playing with the Seafarers expansion, have extra ships available
that you will eventually use to build a surprise settlement, or
better yet a surprise settlement on a new island that gets you a
victory point bonus, or best of all combine with leaping to longest
road. But of course try not to make it obvious you're planning this.

Of course, if you have a hand of 7 unplayed development cards, you will be greatly feared so all the hidden strength tactics involving development cards work best when you have 2-3 unplayed cards.
Perhaps the most important hidden strength:
Gear your initial settlement placement and early building towards the end game. In other words - go for ore and grain, with a little sheep thrown in. You will fall behind in the beginning but so long as you get at least 1 additional settlement, you'll have no problems towards the end of the game while others stall out. Taken to extremes, it's possible to win with only 2 settlements, as I did here (nobody realized how strong I was until the game was just about over - those 15 development cards were all mine, though I didn't have enough turns to play them all!):