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

[Formendacil]

Originally Posted by Legate of Amon Lanc

Nevertheless, on top of everything: Does Merry actually take this one key with him somewhere abroad, to Isengard, Gondor...? (Okay, he probably loses it somewhere along the way, but maybe at least as far as Rivendell?)

Maybe, but as I read your post, I was given to suspect a different answer:

Originally Posted by Legate of Amon Lanc

Last remark belongs to the often forgotten Fatty Bolger.

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.

[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...

[Inziladun]

Originally Posted by Legate of Amon Lanc

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...

He wouldn't really have needed it though, would he? Saruman (according to non-canon UT ) had been in the Shire in earlier times, probably going over Sarn Ford. Why bother with the Old Forest in that case?

There were a lot of seeming opportunities for that key to have been lost. Maybe it's lucky Frodo and Co. didn't need it to enter the Shire covertly when they returned; though maybe had they known of the presence of the Ruffians, the thought could have been entertained.