LotR -- Book 1 - Chapter 06 – The Old Forest

[Legate of Amon Lanc]

Originally Posted by Formendacil

It seems most likely to me that Merry would have left the key with Fatty--not least if it was either a.) one of only a very few keys and/or b.) borrowed from someone else.

That actually is not possible. They say goodbye to Fatty, then they ride into the tunnel which runs under the hedge, then Merry unlocks the gate and locks it behind them ("the sound was ominous"). That's in fact what made me pay attention to it in the first place - the realisation that Merry locked the gate behind them, therefore, he had to keep the key all the way through their journey.

Now I am just thinking where else he might have lost it along the way - and came to the conclusion that if he managed not to lose it somewhere randomly (slipping on the slope of Caradhras, for example), he would have lost it at the latest when the Uruk-hai captured him. Wow, just imagine. Pretty handy for Saruman had they managed to get it to Isengard; it would have been actually quite fitting if ol' Sharkey got the key...