As with the 1893-S Quarter Dollar, the 1894-S is found with the mintmark in two positions: one with the S above the space between the R of QUARTER and the D of DOLLAR and another with the S above the left edge of the D. Examples of both can be seen by clicking on the "View All Images" link.
Sources and/or recommended reading:
"Those Shifty Mintmarks" by David Lange, The Numismatist, September 2010, pp. 21-22
Heritage 5/2000:7324 - Eugene H. Gardner Collection, Part IV - Heritage 10/2015:98556, $12,925
Pinnacle Rarities, sold privately in 1/2008 - Eugene H. Gardner Collection - Heritage 5/2015:98399, $17,625
American Numismatic Rarities 10/2004:345, $7,000