Firstly "their" and "they" can be singular or plural. Context should determine which it is, and when talking about a single pair of glasses it should be clear that there is only one owner.
Regarding your question of whether it is correct to assume female gender - this is unusual as historically the masculine gender has been the default when speaking about an unidentified person. Cultural changes mean this may be on the decline, but most will agree this is the norm at least for now.
I can only assume if a person used the female gender to describe the unidentified owner of a lost pair of glasses that the style or design of the glasses was in some way feminine!