Ferrets can have exactly the same markings as a Polecat. It is likely you saw an escapee and if it was a large Hob (male) then it's an easy mistake. I once owned a Polecat and he was very similar to ferrets I also kept at the time, just bigger and softer as in woss!
Plenty of ferrets are abandoned. Typical scenario is the ferret is released loose down a burrow, catches a rabbit, kills and eats some of it then has a big kip. Only solution is to dig it out. That can be impossible due to tree roots and perhaps a very deep burrow. So matey has little choice but to go home and maybe return next day for a look around.
The two can look very much the same. Check this for more info...
http://jncc.defra.gov.uk/page-2795
Chris.