My 77 mom has stm loss, but now it appears to be worst, misplacing her keys/purse in the house. she also thinks others are there. causes?

Delirium?: If this is a quite new finding, i would query whether she is ill. Older people are more frail and will often times get delirium with new infection (eg, pyelonephritis, pneumonia, etc). This can be worsened if there is baseline dementia. The treatment for delirium is to treat the disease that caused it (ie, antibiotics for the above). I would take her to the doctor.