The treasure, the criminals who stole it and disappeared in a small steam launch, cannot be located. So Holmes is 'morose'. It occurred to me I had no idea of that word's etymology.From AHD:
The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.

morose

SYLLABICATION: mo·rose
PRONUNCIATION: m-rs, mô-
ADJECTIVE: Sullenly melancholy; gloomy.
ETYMOLOGY: Latin mrsus, peevish, from ms, mr-, self-will, caprice, manner. See m-1 in Appendix I.
OTHER FORMS: mo·rosely —ADVERB
mo·roseness —NOUN

That is not clear. Is it derived from 'morsus' = bite?
My impression of 'morose' is discouraged,depressed, gloomy.

I think the problem is that the vowels with macrons have disappeared. Here is Lewis and Short on morosus:

http://makeashorterlink.com/?V315264C6They seem to be citing Cicero as evidence for the link to mos -- custom, manner

Bingley