I noticed this in my garage on the inside trim of a backdoor. It seems to have been painted over several times. I would say it is a mezuzah, except:
- It is very small
- The lettering seems off
- It is inside
- It is not at an angle
- It is on the trim rather than the jamb
- It is on the left side of the door
I am not sure how many of these are rules, and some (or all) may very well be nothing more than my observations.
EDIT:
Just to sum things up:
- The size does not matter
- The lettering on the case doesn't matter much, may have deteriorated, or be poorly done
- Inside and on the trim is fine, provided it is close to the jamb
- Straight vertical rather than tilted is a sephardic custom
- On the left inside would put it on the right from the outside
It looks like this is just a very small, slightly abnormal case that is hung in an uncommon location/manner. When I have time to paint over where it was, I will take it off to try to see if there is a scroll inside.