You might have heard that, SharePoint farm solutions are always deployed to GAC, while sandbox webpart solutions are deployed to 'Solution Gallery' at the site collection level (under '_catalogs/wp' url). So the question is, where does the assemblies of a sandbox solution gets deployed to? Initially my assumption was that, it was getting deployed to content DB. After searching on this, I came to stumble on this insightful article which briefs on this very same question. In the article the default location for sandboxed assemblies is given as 'C:\ProgramData\Microsoft\SharePoint\UCCache'. The author specifically mentions that, this path is configurable by admin. So don't get confused, if you don't find any assemblies in the above location when the sandbox code is executing.
At last, got my query resolved.