Monk eventually realizes the kidnappers weren't interested in the grandmother at all, but rather the antique fanback Windsor chair she was sitting in.
The episode you are looking for is Season 2, Episode 13, titled " Mr. Monk and the Missing Granny " . Episode Synopsis Watch Monk S02E 13 1
The chair was personally handcrafted by Thomas Jefferson in 1774 and is worth over two million dollars. Where to Watch Watch Monk S02E 13 1