Another technique for unknown distant DNA matches on Ancestry:
1) Use the "custom groups" feature to create a color-coded group for each of your ancestral lines.
2) Assign each of your *known* relatives whom you've matched with on Ancestry to a group or groups, identifying which line/s they descend from. (One match can belong to multiple groups.)
3) When you look at an unknown match, go to the list of "Shared Matches" with that person and click on "Groups." The drop-down will show you if the person matches predominantly with your known relatives from a specific ancestral line.
4) Research whether the unknown match shares any of your towns or surnames on that line.
I've used this method to identify quite a few unknown Ancestry matches. It seems to work even with pretty distant relationships.
For instance, I looked at one unknown "5th-8th cousin" with whom I shared only 14 cms — but with whom I had 3 already-identified shared matches, all of whom came from the same line of my family tree. This 14-cm match turned out to indeed be a 4th cousin from that line of my family, The technique also seems to work especially well when— as in this case — the unknown match is only part-Jewish.