When I trained as a nurse, one of the things we had to ask about when admitting a patient was whether they were on any medication, and the doctors had to check too, and then had to write the patient up for all their medication on their drug charts. The medicines were dispensed by the hospital (and the patient's relatives took their medication home), and the nurses dispensed all medications.
Two members of the nursing staff had to do the drugs round together, checking the patient's drug chart, ensuring we gave all the medication due at that time, checking we'd dispensed the correct medication and dose, watching the patient take it, then signing the chart so we could prove it had been done properly.
Under NO circumstances would a patient's medication have been left on their locker - it's just not safe there. A patient who is unwell and stressed won't necessarily remember to take their drugs, and any drugs administered should be overseen by the ward staff in case of any potential problems with combinations of drugs. Also, drugs left on a locker could be taken by the wrong patient or stolen.
I am so sorry about what happened to your mother, Fluffy - in my opinion it is a clear failure of care by the hospital, and an easily preventable one too.