I too can only find references to the Sind Horse, which was first raised about 1830.
If his papers give no details it would probably indicate a short term assignment during a temporary emergency. It was not uncommon for Senior NCOs to be attached to part-timers for training and discipline.
The only sure way of finding the duration at least would be to hope that the Muster and Pay Books survive in Kew (in series WO12/16, WO = War Office) for the Worcester Regiment. These are books listing details of the monthly muster (roll-call) and quarterly payments made to Other Ranks. Any man not with the regiment - sick, on leave, temporary assignments etc - will be listed. There could also be details of the travel expenses he would have received if applicable. And you may find other men on assignment as well.
The problem is that these start to disappear during the 1880s, ie the War Office threw them away. I believe the latest to survive is about 1895, so you may be lucky. But they are original documents, so you will have to visit Kew or employ a researcher.
Ken
PS: I have searched for "sind volunteers" in TNA catalogue -
http://www.nationalarchives.gov.uk/catalogue/search.asp?j=1 - and it comes up blank. If you can scan and post the document that mentions the outfit you could get a second opinion.